an applied guide to process and plant design

AN APPLIED GUIDETO PROCESS ANDPLANT DESIGN

For Annemarie

AN APPLIED GUIDE TOPROCESS AND PLANTDESIGN

SEÁN MORAN

AMSTERDAM • BOSTON • HEIDELBERG • LONDON

NEW YORK • OXFORD • PARIS • SAN DIEGO

SAN FRANCISCO • SINGAPORE • SYDNEY • TOKYO

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CONTENTS

Preface xi

Acknowledgments xii

Part 1 Practical Principles

Introduction 3

1. Process Plant Design 5

Introduction 5

What is engineering? 5

What is design? 6

Engineering design 7

Project life cycle 8

Process plant design 9

Process plant versus process design 11

Academic versus professional practice 13

State of the art and best engineering practice 18

The use and abuse of computers 19

Further reading 20

2. Stages of Process Plant Design 21

General 21

Conceptual design 21

“Conceptual design of chemical processes” 23

Front End Engineering Design (FEED)/basic design 25

Detailed design 26

Site redesign 27

Posthandover redesign 28

Unstaged design 29

Product engineering 29

Fast-tracking 30

Further reading 33

3. Process Plant Design Deliverables 35

Overview 35

Design basis and philosophies 35

v

Specification 36

Process Flow Diagram (PFD) 37

Piping and instrumentation diagram 38

Functional Design Specification (FDS) 40

Plot plan/general arrangement/layout drawing 40

Program 42

Cost estimate 42

Equipment list/schedule 45

Datasheets 46

Safety documentation 47

Design calculations 48

Isometric piping drawings 51

Simulator output 52

Further reading 52

4. Twenty-First Century Process Plant Design Tools 53

General 53

Use of computers by chemical engineers 54

Implications of modern design tools 54

Categories of design 56

Tools—Hardware 57

Tools—Software 59

Further reading 68

5. The Future of Process Plant Design 69

Process porn 69

Will first principles design replace heuristic design in future? 71

Will process design become a form of applied mathematics in future? 72

Will primary research become the basis of engineering design in future? 72

Will “chemical process design” replace process plant design in future? 72

Will network analysis form the core of design practice in future? 73

Will process simulation replace the design process in future? 73

Will process plant design never change? 74

Further reading 75

Part 2 Professional Practice

6. System Level Design 79

Introduction 79

How to put unit operations together 79

Matching design rigor with stage of design 80

vi Contents

Implications for cost 81

Implications for safety 82

Implications for robustness 82

Rule of thumb design 83

First principles design 83

Design by simulation program 84

Sources of design data 84

Further reading 87

7. Professional Design Methodology 89

Introduction 89

Design methodologies 90

The “is” and “ought” of process design 91

Right versus wrong design 92

Interesting versus boring design 92

Continuous versus batch design 94

Simple/robust versus complicated/fragile design 98

Setting the design envelope 100

Implications of new design tools 102

Importance of understanding your design 103

Manager/engineer tensions in design 103

Whole-system design methodology 105

Design stages in a nutshell 106

Variations on a theme 107

Further reading 107

8. How to Do a Mass and Energy Balance 109

Introduction 109

Handling recycles 111

How to set it out in Excel 112

Using Excel for iterative calculations: “Goal Seek” and “Solver” 113

9. How to Do Hydraulic Calculations 115

Introduction 115


Hydraulic networks 121

Pump curves 122

Further reading 126

viiContents

Part 3 Low Level Design

10. How to Design and Select Plant Components and Materials 129

Introduction 129

What process engineers design 129


Materials of construction 131

Mechanical equipment 138

Electrical and control equipment 145

Further reading 151

11. How to Design Unit Operations 153

Introduction 153


Rule of thumb design 153

Approaches to design of unit operations 154

Sources of design data 156

Scale-up and scale-out 156

Neglected unit operations: separation processes 157

Further reading 161

12. How to Cost a Design 163

Introduction 163


The basics 164

Academic costing practice 165

Professional costing practice 167

Further reading 171

Part 4 High Level Design

13. How to Design a Process Control System 175

Introduction 175


Operation and Maintenance manuals 176

Specification of operators 177

Automatic control 177

Standard control and instrumentation strategies 180

Further reading 199

viii Contents

14. How to Lay Out a Process Plant 201

Introduction 201

General principles 202

Factors affecting layout 204

Plant layout and safety 208

Plant layout and cost 209

Plant layout and aesthetics 210


Further reading 216

15. How to Make Sure Your Design Is Reasonably Safe and Sustainable 217

Introduction 217

Why only reasonably? 217


Conceptual design stage 219

Detailed design stage 221

Formal methods: safety 222

Formal methods: sustainability 229

Specification of equipment with safety implications in mind 230

Specification of safety devices 235

Types of safety device 235

Further reading 245

Sources 245

Part 5 Advanced Design

16. Professional Practice 249

Introduction 249

General design methodology 249

Informal design reviews 250

Formal design reviews 251

Quality assurance and document control 252

Informal data exchange 253

Further reading 254

17. Beginner’s Errors to Avoid 255

Introduction 255

Lack of equipment knowledge 259

Lack of knowledge of many types of unit operations 261

Lack of knowledge of many materials of construction 262

ixContents

Lack of utilities 262

Layout 262

Process control 263

Further reading 264

18. Design Optimization 265

Introduction 265


Indicators of a need to integrate design 267

How to integrate design 268

When and how not to integrate design 274

Where’s the harm? The downside of academic “process integration” 274

Further reading 275

19. Developing Your Own Design Style 277

Introduction 277

The art of engineering 277

The philosophy of engineering 278

The literature of engineering 279

The practice of engineering 279

Personal Sota 280

Further reading 282

Appendix 1: Integrated Design Example 283

Appendix 2: Upset Conditions Table 289

Appendix 3: Plant Separation Tables 301

Appendix 4: Checklists for Engineering Flow Diagrams 323

Appendix 5: Teaching Practical Process Plant Design 347

Glossary 367

Index 369

x Contents

PREFACE

I am a highly experienced practical professional process engineer who has

designed, commissioned, and undertaken troubleshooting of many process plants, and

mentored and trained many other professional engineers in how to do these things.

I am also a university professor who has taught process plant design to undergraduates

and postgraduates for a number of years. This has required me to reflect upon what

I know about the subject, how I know it, and how I can teach it to someone else. In

this book I will assume the role of the experienced engineer, who takes lucky gradu-

ate chemical engineers by the hand in their first job or two and shows them what

engineering is really about. Many are not so lucky as to have expert guidance. In

doing so I will take the fairly informal tone I do when undertaking that task, and may

on occasion express my frustration with the way the subject is taught in UK (and to

the best of my knowledge worldwide) higher education. After all, it is not possible to

describe how something might be improved without acknowledging that the present

situation is less than perfect. I may also express the odd opinion and, as this is a distil-

lation of experience rather than a scientific paper, I may not necessarily offer a refer-

ence to a peer-reviewed journal article in support of these opinions. However (despite

the informal style of writing), the more controversial or provocative an opinion

expressed, the more effort I have put into making sure that it is held by the majority

of professional process plant designers. Toward this end, this book was reviewed by a

panel of 40 professional engineers across sectors and worldwide. Many of the ideas

which seem controversial in academic circles have been the subject of articles in The

Chemical Engineer magazine, where they have been met with universally positive

professional comment. The foundation of this book is practice, not theory.

Throughout the text I will, however, offer quotations from others, links to books and

even, on occasion, primary literature. These should not be misunderstood as the basis

of my opinions. In the case of quotations, I am simply quoting people who agree

with me. Suggestions for Further Reading are referenced to avoid my having to

reproduce the content of these often weighty books, or reinvent the wheel.

xi

ACKNOWLEDGMENTS

I would like to thank all of those who have helped me, especially my wife,

Annemarie. Her patient assistance and sure hand with language have been essential in

the preparation of this book. Heartfelt thanks also to Pat Cunningham for his pains-

taking proof-reading and advice on comma usage!

My fellow engineers have given generously of their time in helping me to make

sure that what I have written represents consensus opinion, most notably: Gareth

Brown, Chris Davis, Harvey Dearden, Mike Dee, Carlos Harrison, Myke King,

Liangming Lee, Jim Madden, Brian Marshall, Keith Plumb, Ken Rollins, Michael

Spreadbury, and Alun Rees. Thanks also to my colleagues George Chen and Giorgios

Dimitrakis at the University of Nottingham for their comments and advice.

I am grateful to those who have kindly allowed me to reproduce images and mate-

rial, including Tony Amato at Doosan Enpure, Sophie Brouillet at AMOT, John

Evans at the Olympic Delivery Authority, Claudia Flavell-White at the IChemE,

Kerry Harris at AUMA, Ernest Kochmann at Newson Gale, Malcolm Ledger at

Lechler, Stuart Leigh and Ian Andrews at SLR Consulting, Edward Luckiewicz,

Fiona Macrae at Crowcon, Glenn Miller at Grundfos, Ross Philips at the EEMUA,

Jennifer Reeves at Elfab, Henry Sandler, Tosh Singh at Lutz-Jesco (GB) Ltd., Mike

Wainwright at Ascendant and Kirsty Warren at WRAP.

The staff at Elsevier, especially Cari Owen, Lisa Jones and Fiona Geraghty, have

been invaluable in making my experience as a neophyte to book publishing surprisingly

painless.

xii

PART 1

Practical Principles

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Introduction

Process plant design is the pinnacle of chemical engineering design. Chemical engineer-

ing was developed based on the insight by Davis that all process industries used similar

unit operations, which could be understood using sector independent analytical tools.

Many “design tools” now commonly taught in academia incorporate assumptions that

imply that all chemical engineering design is for the petrochemical sector, but chemical

engineering has encompassed food and drink, inorganic chemical manufacture, and so

on since its inception.

This book is about Process Plant design and, while examples may be drawn from

my personal experience in the water and environmental sectors, it is intended to reflect

consensus practice across the broad discipline. The IChemE’s “Chemical Engineering

Matters” discussion document identifies the energy, water, food and nutrition, health,

and well-being sectors as the future of Chemical Engineering. We need to avoid confu-

sion between chemical engineering and petrochemical engineering, a small and argu-

ably diminishing subset of the discipline.

I have spoken and corresponded with hundreds of process engineers, most notably

in the United Kingdom, Middle and Far East, in all industries to verify that the

approaches suggested in this book do represent current consensus practice. It seems that

professional practice has not changed very much over the last couple of decades, but

that what is taught in universities has drifted further and further from professional prac-

tice during that period.

I am writing this book because I think that Chemical Engineering Education has

lost its way, and become too theoretical and abstract to adequately serve its purpose,

namely to provide the “academic formation of a Chartered Chemical Engineer” as the

UK’s IChemE puts it in its course accreditation guidelines.

The book is based on material I have delivered as part of the design courses I teach at

the University of Nottingham, which is in turn based in my continuing professional

engineering practice and professional training of my fellow engineers. It should be of use

to undergraduate and postgraduate students, as well as early-career process plant designers

and to university lecturers who wish to teach a more realistic version of plant design.

The book is in five parts. Firstly, I explain what process plant design is and how it is

done in broad terms, then I give advice on professional practice in the most important

aspects of process plant design, in general, and then at low and at high levels, and lastly

I cover more advanced aspects of design.

It should be noted that this is a book about process plant design, rather than what is

known nowadays in research-led universities as process design. There is no such thing as

process design—processes happen in plants, and plants are the things which engineers design.

3

It has become clear in writing this book that we as a profession unhelpfully use the

same words to mean different things. The meaning of the words and phrases

Conceptual Design; HAZOP; Functional Design Specification; Design Philosophy;

Design Basis; Process Intensification; Process Design; Optimization; Reproducibility;

Repeatability; and Precision were particularly contested.

I have explained the sense in which I have used these words in the text at the point

of first using them, and have included a glossary at the end. I am not claiming that my

usage is the only correct one, but I have used them consistently in the book, and reflect

to the best of my knowledge the most common meaning.

Sean Moran

2014

4 An Applied Guide to Process and Plant Design

CHAPTER 1

Process Plant Design

INTRODUCTION

Whilst this may not be as obvious to today’s students of the subject as it should be,

chemical engineering is a kind of engineering, rather than a branch of chemistry.

Similarly, professional engineering design practice has next to nothing to do with

the thing called process design in many university chemical engineering departments.

I will cover the reasons for this elsewhere, but first let’s start by dispelling some

confusion, by clearing up what engineering is (and is not), and what design is all about.

WHAT IS ENGINEERING?

I still feel glad to emphasize the duty, the defining characteristic of the pure scientist—probably to be found working in universities—who commit themselves absolutely to special-ized goals, to seek the purest manifestation of any possible phenomenon that they are investi-gating, to create laboratories that are far more controlled than you would ever find inindustry, and to ignore any constraints imposed by, as it were, realism.

Further down the scale, people who understand and want to exploit results of basicscience have to do a great deal more work to adapt and select the results, and combinethe results from different sources, to produce something that is applicable, useful, andprofitable on an acceptable time scale.

C.A.R. Hoare

Engineers are those people “further down the scale” as Hoare the classicist and

philosopher puts it, although I disagree that we “exploit the results of basic science.”

Our profession stands on other foundations, though you may have been taught some-

thing different in university.

In academia there is almost universal confusion between mathematics, applied

mathematics, science, applied science, engineering science, and engineering. Allow

me to unconfuse anyone so confused before we get started:

Mathematics is a branch of philosophy. It is a human construction, with no empir-

ical foundation. It is made of ideas, and has nothing to do with reality. It is only

“true” within its own conventions. There is no such thing in nature as a true circle,

and even arithmetic (despite its great utility) is not factually based.

Applied mathematics uses mathematical tools to address some real problem. This is

the way engineers use mathematics, but many engineers use English too. Engineering

is no more applied mathematics than it is applied English.

An Applied Guide to Process and Plant Design r 2015 Elsevier Inc.All rights reserved. 5

Science is the activity of trying to understand natural phenomena. The activity is

rather less doctrinaire and rigid than philosophers of science would have us believe,

and may well not follow what they call the scientific method, but it is about explain-

ing and perhaps predicting natural phenomena.

Applied science is the application of scientific principles to natural phenomena to

solve some real-world problem. Engineers might do this (though mostly they do not)

but that doesn’t make it engineering.

Engineering science is the application of scientific principles to the study of engi-

neering artifacts. The classic example of this is thermodynamics, invented to explain

the steam engine, which was developed without supporting science.

Science owes more to the steam engine than the steam engine owes to Science.L.J. Henderson

This is the kind of science which engineers tend to apply. It is the product of the

application of science to the things engineers work with, artificial constructions rather

than nature.

Engineering is a completely different kind of thing from all preceding categories.

It is the profession of imagining and bringing into being a completely new artifact

which achieves a specified aim safely, cost-effectively, and robustly.

It may make use of mathematics and science, but so does medicine if we substitute

the congruent “medical science” for “engineering science.” If engineering was simply

the application of these subjects, we could have a more-or-less common first and sec-

ond year to medical and engineering courses, never mind the various engineering

disciplines.

Now that we are clear about what engineering is, let us consider what design is.

WHAT IS DESIGN?

Rather than being some exotic province of polo-necked professionals, the ability to

design is a natural human ability. Designers imagine an improvement on reality as it is,

we think of a number of ways we might achieve the improvement, we select one of

them, and we transmit our intention to those who are to realize our plan. The docu-

ments with which we transmit our intentions are, however, just a means to the

ultimate end of design—the improvement on reality itself.

I will discuss in this book a rather specialized version of this ability, but we should

not lose sight of the fact that design is in essence the same process, whether we are

designing a process plant, a vacuum cleaner, or a wedding cake.

Designers take a real-world problem on which someone is willing to expend

resources to resolve. They imagine solutions to that problem, choose one of those

solutions based on some set of criteria, and provide a description of the solution to


the craftsmen who will realize it. If they miss this last stage and if the design is not

realized, they will never know whether it would have worked as they had hoped.

All designers need to consider the resource implications of their choices, the like-

lihood that their solution will be fit for the purpose for which it is intended, and

whether it will be safe even if it not used exactly as intended.

If engineers bring a little more rigor to their decision making than cake designers,

it is because an engineer’s design choices can have life and death implications, and

almost always involve very large financial commitments.

So how does engineering design differ from other kinds of design?

ENGINEERING DESIGN

Engineering problems are under-defined, there are many solutions, good, bad and indifferent.The art is to arrive at a good solution. This is a creative activity, involving imagination, intui-tion and deliberate choice.

Ove Arup

Like all designers, design engineers have to dream up possible ways to solve pro-

blems and choose between them. Engineers differ from, say, fashion designers in that

they have a wider variety of tools to help them choose between options.

Like all designers, the engineer’s possible solutions will include approaches to simi-

lar or analogous problems which they have seen to work. One of the reasons why

beginners are inferior to experts is their lack of qualitative knowledge of the many

ways in which their kind of problems can be solved, and more important still, those

ways which have been tried and found wanting.

Engineers need to make sure they are answering the right question. For example,

a UK missile program called “Blue Streak” was a classic engineering failure because

the problem was not correctly stated. It was designed to be a long-range missile for

nuclear warheads, but the missile had to be fuelled immediately before launch and

it took thirty minutes to do this. Hence the missile was useless for the intended

purpose, as it was not capable of sufficiently rapid deployment.

In “To Engineer is Human”, Henry Petroski discusses the importance of avoiding

failure in engineering design. Many of his examples of failure, however, were caused

not by misspecification, but by designers who forgot that the models used in design

are only approximations, applicable in a fairly narrow range of circumstances.

Billy Vaughn Koen goes still further toward the truth, when he points out that “all

is heuristic.” Even arithmetic is a heuristic. There are no absolute truths in mathe-

matics, science, or engineering. There are only approximations, probabilities, and

workable approaches. Engineers may just be a little clearer about these issues than

mathematicians and scientists, because our solutions absolutely have to work.

7Process Plant Design

PROJECT LIFE CYCLE

The niche or niches into which process plant design fits exist in the wider background

of an engineering project life cycle. Engineers conceive, design, implement, and operate

(CDIO) engineering solutions, as the CDIO Initiative points out.

The details of project life cycles vary between industries, but there is a common

core. For example, here is the product life cycle for a pharmaceutical project:

1. Identify the problem—this stage is frequently overlooked because people think

they know what the problem is. In reality, many solutions are for problems that

do not exist (academic research often focuses on finding solutions without associ-

ated problems).

2. Define the problem in business, engineering, and scientific terms—often done

poorly with the problem again being defined in terms of perceived solutions.

3. Generate options that provide potential solutions to the problem.

4. Review the options against agreed selection criteria and eliminate those options

that clearly do not meet the selection criteria.

5. Generate the outline process design for the selected options.

6. In parallel:

a. Commence development work at laboratory scale to provide more data to

refine the business, engineering, and scientific basis of the options.

b. Commence an engineering project to evaluate the possible locations, project

time scale, and order of magnitude of cost.

c. Develop the business case at the strategic level.

7. Based on the outcomes of step 6, reduce the number of options to those carried

forward to the next level of detail.

8. In parallel:

a. Continue the development work at the pilot plant scale.

b. Based initially on the data from the laboratory scale, develop the design of the

remaining options to allow a sanction capital cost estimate to be generated

and a refined project time scale.

c. Continue to develop the business case leading to a project sanction request at

the appropriate corporate level.

9. Based on the outcomes of step 8, select the lead option to be designed and installed.

10. In parallel:

a. Continue the development work at the pilot scale.

b. Carry out the detailed design of the lead option. A “design freeze” will almost

certainly need to occur before the development work is complete.

11. Construct the required infrastructure, buildings, etc. and install the required

equipment.

12. Commission the equipment.


13. Commission the process and verify that the plant performs as designed and

produces product of the required quality.

14. Commence routine production.

15. Improve process efficiency based on the data and experience gained during

routine production.

16. Increase the plant capacity by making use of process improvements and optimiza-

tion based on the data and experience gained.

17. Decommission the plant at the end of the product life cycle.

The pharma sector tends to run more stages in parallel than other sectors for

reasons discussed later, but most of these stages exist in all sectors.

Where does design fit into this? Consultants might call stages 1�3 above plant

design. Those with a background in design and build contracting, like me, usually

think of design as being predominantly what those in operating companies call “grass-

roots design,” broadly stages 3�10 above. Those who work for operating companies

might call stages 15 and 16 plant design.

I suppose an argument can be made for all the above, but it should be noted that

before step 14, very limited design information is available. The “design tools” popu-

lar in academia are used professionally only for stage 15/16 plant design, rather than

stage 3�10 “grassroots” plant design for this reason.

I am going to use the term “process plant design” in the sense of stage 3�10

design throughout this book, even though stages 1�3 and stages 15 and 16 are

certainly related fields of professional activity which I have been involved in. This is

because this kind of design is closest to the meaning of the terms outside chemical

engineering and there are several reasons for this.

Firstly, a piece of UK legislation called the Construction Design and Management

(CDM) regulations require a person or company to declare themselves the designer of

a plant, responsible for the safety implications of the design. This designer is almost

always the entity responsible for stage 3�10 design.

Secondly, the process guarantee is almost always offered by the company responsi-

ble for stage 3�10 “grassroots” design.

Thirdly, this definition of design is that used by more or less everyone involved in

design activity other than chemical engineers.

Fourthly, it’s my book, and there are already lots of books on stage 15/16 “process

design.” This is, as far as I know, the only current book on “grassroots” process

plant design.

PROCESS PLANT DESIGN

In researching this book, I looked at what had been published in texts intended to

describe a process plant design methodology. There were promising sounding books


with titles like “The art of. . .,” “A strategy for. . .,” “Systematic methods for. . .,”“Design of simple and robust process plants,” etc. I know that students and early stage

designers lack an understanding of these things, as well as a gestalt of systems, but the

overwhelming majority of these books failed to meet the promise of their titles.

Process plant design is an art, whose practitioners use science and mathematics,

models and simulations, drawings and spreadsheets, but only to support their profes-

sional judgment. This judgment cannot be supplanted by these things, since people

are smarter than computers (and probably always will be). Our imagination, mental

imagery, intuition, analogies and metaphors, ability to negotiate and communicate

with others, knowledge of custom and practice and of past disasters, personalities, and

experience are what designers bring to the table.

If more people understood the total nature of design they would see the futility of

attempts to replace skilled professional designers with technicians who punch numbers

into computers. Any problem a computer can solve isn’t really a problem at all—the

nontrivial problems of real-world design lie elsewhere.

Engineering problems will almost certainly always be far quicker to solve by asking

an engineer, rather than by programming a computer, even if we had the data

(which we can never have on a plant which hasn’t yet been built), a computer smarter

than a person, and a program which codes real engineering knowledge, instead of a

simplified mathematical model with next to no input from professional designers.

I wonder how the medical profession would feel if scientists and mathematicians

suggested, without consulting medics, that they could produce an expert system

which would exceed the competence of doctors?

This is a classic academic purist’s mistake: The psychologist claims that sociology is

just applied psychology; the biologist says that psychology is just applied biology, the

chemist that biology is just chemistry with legs, the physicist that chemistry is just

applied physics, the mathematician that physics is applied mathematics, and the philos-

opher that mathematics is applied philosophy. Emergent properties are irrelevant to

the theorist, but in practical matters they may be everything.

Donella Meadows explains, in “Thinking in Systems,” an intuitive system level

view which is identical in many ways to the professional engineer’s view. We share

this view with the kindergarteners who also excel at a design exercise called the

“Marshmallow Challenge” which I use in my teaching (see Further Reading).

The roots of this system level view are natural human insights, which we may be

educating out of our students.

Meadows explains that she makes great use of diagrams in her book because the

systems she discusses, like drawings, happen all at once, and are connected in many direc-

tions simultaneously, whilst words can only come one at a time in linear logical order.

Process plant design is system level design, and drawings are its best expression—

other than the plant itself—for the same reasons given by Meadows.


PROCESS PLANT VERSUS PROCESS DESIGN

Experimental scientists today, despite Einstein and Darwin, seem loath to abandon the searchfor an eternal changeless unhistorical reality of which pure mathematics could be the model.

Gordon Childe

An inward-looking school of “process design” as a form of applied mathematics

has arisen in elite research institutions, whose practitioners collaborate only with their

fellow researchers. They build upon each other’s work, but their outputs are not used

by, or indeed of use to, the profession.

Extending this philosophy to teaching programs, many universities have replaced

essential professional knowledge with modules in which students learn to use

researcher software so that, later in the course, they can carry out “process design” as

these researchers do it.

Adherents of this school of thought argue that it is the job of industry to produce

engineers, whilst academia’s job is to provide an education in applied mathematics.

They would argue that, not only should we follow institutions such as Tokyo

University in not teaching our students to read engineering drawings, but they should

not even spend much time learning about science.

The prevalence in academia of this approach based on modeling, simulation, and

mathematical techniques such as network analysis seems to some extent to be an

artifact of the way research is funded. Research which takes place in a PC rather

than a laboratory (or worse yet a pilot plant) is relatively cheap to conduct, and thus

to fund.

It is of course a valid function of engineering research departments to develop

new design methodologies. Some aspects of these approaches may have niche applica-

tions in professional practice, but the overwhelming majority will not be taken up by

the profession, as they do not help the professional to achieve their aims.

It is inevitable that researchers will consider their work important, even vital, but

if it is not of use to the profession, it is research material of academic interest only. In

my view, its most useful place (if any) within the UK model of chemical engineering

education is only during the masters’ year, when the Institution of Chemical

Engineers (IChemE) requires students to receive greater exposure to research.

Variation and selectionVariation/creativity

When I examined myself and my methods of thought, I came to the conclusion that the giftof fantasy has meant more to me than my talent for absorbing positive knowledge.

Einstein

The engineering design process consists of the generation of candidate solutions,

and of then selecting from these those most likely to be safe, cost-effective, and robust.


Coming up with the candidates is a creative process, involving the use of imagina-

tion, analogy from natural or artificial systems, knowledge of the state of the art, and

so on. Selection of candidates is, however, often more of a grind.

Engineers will almost always make use of mathematical tools to help them with

selection, though the academic study of engineering can overemphasize these tools,

which are supposed to inform professional judgment rather than supplant it.

In “Engineering and the Mind’s Eye,” Ferguson points out a certain intellectual

snobbery common in academia which values the mathematical over the verbal, and

both of these over the visual, to the detriment of what we might call visual intelli-

gence. Chemical engineering students may leave university with little or no drawing

ability, or any development of their “visual intelligence.”

It is left by many academics to their employers to teach new engineers this essential

part of their skillset. This is bad enough, but perhaps the greatest problem with engi-

neering education is slightly broader: the lack of opportunity to exercise creativity.

In a group design exercise previously referred to, known as the “Marshmallow

Challenge,” kindergarten children have been shown to outperform many engineering

graduates in a test of practical imagination, visual reasoning, feel for materials, and

group work.

I used to teach my students a risk management tool called Hazard and Operability

Study (HAZOP; described later in this book) early in their course of study. All of

them could master the formal methodology, but very few indeed could realistically

imagine what might happen if a component failed (I have included at Appendix 2 a

table to help with this).

Similarly, we used (as many institutions still do) to teach students how to use

modeling and simulation programs such as Hysys in place of teaching process design.

The vast majority of our students never learned to carry a mental model of a complex

system in their heads, nor did they understand the limitations of the programs. They

were Hysys operators, not engineers. This is not to say that modeling and simulation

programs are worthless, but that they are being misused in academia, to the detriment

of our students’ understanding.

There is some evidence to suggest that this is a subset of a more general problem,

in which the arguably too-ready availability of IT means that less is held in memory

by younger people, and their visualization and mental modeling abilities are suffering

as a result.

There are many formal systems intended to enhance creativity, but there is little

evidence to suggest that they do more than ensure that a larger number of approaches

are considered than might be if a less formal approach was followed. I find in my

teaching that a greater degree of life experience is a better predictor of number of

candidate solutions generated than the degree of adherence to a formal creativity

enhancement methodology.


Selection/analysisModeling and simulation programs only address this part of the design process, and

may allow thoughtless processing of options rather than making a genuine considered

choice between well-understood options. Some say we are making technologists

rather than engineers with this approach.

Tools which help the understanding of a system are good. Tools which allow

bypassing of understanding to get to a decision (even if our understanding is “mere”

intuition rather than science) are not. Our best students will still understand, but the

generality will not and there are risky implications associated with this.

Simulation programs are not usually written by professional chemical engineers.

They are usually written by numerate graduates with no plant design or operating

experience, and they run necessarily simplified mathematical models on machines

with a great deal of less processing power than people. Anyone who has seen the

movie “The Terminator” knows what happens when machines build machines!

Professional engineers validate simulation programs against real plants before giving

their outputs any credence. This is why the use of such programs by professional engi-

neers is usually limited to modification of exiting plant rather than the whole-plant

grassroots design addressed by this book.

ACADEMIC VERSUS PROFESSIONAL PRACTICE

In universities, students mainly practice alone or in small single-discipline groups with

simplified and idealized examples. In order to turn the vague messiness of real engi-

neering problems into a collection of unambiguous tasks, a great deal of data is given,

the problem is very tightly framed, and in a “successful” example (judged by student

satisfaction), it is very clear to students that they are faced with carrying out a number

of the tasks which they were previously trained in.

Though these are very often supposedly group exercises, they are structured so that

students can readily split them into a number of standalone tasks. When we examine

students who have carried out such exercises, it becomes clear that none but the most

able have any understanding of the overall picture—the majority are just grinding

through the textbook method, or operating the program. Neither do they have any

appreciation of the complexity of the substructures of a process plant. The method has

produced neither system-level vision, nor an engineer’s grasp of detailed considerations.

This may be useful as a way to learn design calculation techniques, or to illustrate

basic principles, but this is not how the majority of engineering design is done. Pugh

(see Further Reading) calls these academic approaches “Partial Design,” which he

contrasts with the professional’s “Total Design.”

Students mostly learn their basic science and math from professional researchers,

with a very deep knowledge of a necessarily small area, and in the main a love of


radical novelty. Engineering practice is far more holistic, and those who practice tend

to dislike excessive novelty, which they see as inherently risky.

They also tend to dislike too much prediction of real-world outcomes based on

scientific or mathematical models. They would rather see a full-scale working example

of what is proposed: a model is not the thing itself. Professional engineers design

things that absolutely have to work, and usually cost a great deal of money. Efficacy is

a great deal more important than novelty.

This day-to-day activity of most practicing engineers is what Kuhn might call

“normal design,” where most designs are at best an incremental improvement on what

has gone before, with a correspondingly small potential downside.

Practicing engineers operate in a highly constrained environment, and they under-

stand that many of the problems inherent in a design scenario are simply too complex

or vaguely defined to be analyzed rigorously within the resources available.

The design dimension sees engineering as the art of design. It values systems thinking muchmore than the analytical thinking that characterizes traditional science. Its practice is foundedon holistic, contextual, and integrated visions of the world, rather than on partial visions. Typicalvalues of this dimension include exploring alternatives and compromising. In this dimension,which resorts frequently to non-scientific forms of thinking, the key decisions are often based onincomplete knowledge and intuition, as well as on personal and collective experiences.

Figueredo

Pahl and Beitz’s “Systematic Approach” (see Further Reading) splits the challenges of

a design problem into these two components: uncertainty and complexity. According to

them, if it is neither too uncertain nor too complex to be solved using standard design

tools or methodologies, it is not a genuine problem at all, but it is merely a task.

Standard methods are of no use when there is insufficient data to apply them.

There may well be some “tasks,” such as checking the sizes of unit operations using

heuristics, but these are mathematically trivial, and the product may well be ambigu-

ous. There is no opportunity in professional life to start thinking that engineering is a

branch of applied mathematics, or that a computer can solve engineering problems—

computers only carry out tasks. Common sense is what is needed, and a feeling for

ambiguity, qualitative knowledge, and multidimensional evaluation of options: in

short, professional judgment.

The key to a successful design is to understand the problems just well enough to

be able to predict that the desired outcome will be reliably attainable. Science and

mathematics are certainly tools in the engineer’s portfolio, but they are very often not

the most important ones, and they are not its basis.

Capturing nonscientific information about how past designs of the type being

attempted have performed, and the factors associated with success, is often at least as

important. The information is highly situation-specific, and is essentially a codification

of experience.


The information may be presented as a design manual, or as standards, codes of

practice, or rules of thumb. Such documents allow the very complex situations com-

mon in real-world design to be appropriately simplified, making theoretically impossi-

ble parts of the design practically possible. The main purpose of such documents is to

control the design process such as to constrain innovation within the envelope of

what is known to be likely to work based on experience.

In researching this book, I found books by nonchemical engineers who clearly under-

stand all that I have said so far, but for some reason, the link between professional practice

and academic understanding seems to have been broken entirely in my discipline.

Process plants versus castles in the airAn architect friend tells me that there are two kinds of architects, those who know

how big a brick is, and those who do not. Castles in the air are such stuff as dreams

are made of, but real designs need to be built using things we can buy.

Process engineers do not often make things out of bricks, but we nevertheless

make most of what we build from simple standardized subunits, such as lengths of

pipe, and ex-stock valves, pumps, and so on. These items have types and subtypes

which differ from each other in important ways. Choosing between them is no trivial

matter. The essential but qualitative knowledge required to differentiate between these

things is thought, by those who insist that only pure theory matters, to be beneath the

dignity of universities to teach.

If you do not know the dimensions and characteristics of the standard subunits

from which you build something, your designs will be impractical to build. They will

be likely to be less cost-effective, less robust, and less safe than the output of a more

practically minded designer.

The commercially available components almost always represent the lower limit of

the professional engineer’s resolution. We do not care about the atomic structure

of the metal in our pump, but we care about those of its properties which affect the

overall design.

What we design and what we do notWe don’t tend to design things which we can buy, because having things specially

made costs a lot more than buying stock items, and those who design things guarantee

their efficacy. Engineers like to minimize both costs and risks.

Process engineers tend not to carry out design of mechanical, electrical, software,

civil, or building works. They do, however, have to know about the constraints

imposed on designers in these disciplines, and have a feel for the knock-on cost and

other implications of their design choices.


Engineers mostly put standard components together in fairly standard ways.

This doesn’t sound very clever, until you consider that 32 chess pieces arranged on

64 squares following a set of simple rules can be positioned in 1050 legal ways.

Engineers have to manage a similar level of complexity to that handled by chess

players when designing a process plant. Standard parts and standard methodologies do

not reduce the plant designer’s profession to monkey work. They just make it

humanly possible to reliably produce a working outcome.

In the academic setting, many students are taught engineering science in exer-

cises intended to teach science in context. This is fine, but should not be mistaken

for actually teaching engineering. Such academic exercises, which make engineer-

ing design look like applied science, have been stripped of their true complexity.

Often there is only one “right answer” in such exercises. This is neither engineering

nor design.

Standards and specificationsAs practicing engineers we do not design, we specify. Specifications have the collective experi-ences over many years. They include successes and failures, and ultimately they stop us fromkilling people.

Anonymous Oil and Gas Process Engineer

Standards and specifications exist to keep design parameters in the range where the

final plant is most likely to be safe and to work. They also serve to keep design docu-

mentation comprehensible to fellow engineers. A brilliant design which no one else

understands is worthless in engineering.

There are a number of international standards organizations—ISO in Europe, DIN

in Germany, ANSI, ASTM, and API in the United States, and so on (“British

Standards” in the United Kingdom are now officially a subset of ISO).

Since I work in a UK University, I am going to refer to British Standards in this

book where available, most notably those governing engineering drawings. The use of

British Standards (or any other used consistently and clearly) for drawings reduces the

likelihood of miscommunication between engineers via their most important channel

of information exchange.

The availability of interchangeable standard parts makes much of engineering

design simple in one way, but introduces an extra stage which can often be omitted in

academic practice. After an approximate theoretical design, practitioners redesign in

detail using standardized subcomponents. At this point, the very precise-looking

academic design can turn out to be very precisely wrong.

We do not, for example, use 68.9 mm internal diameter pipe, we use 75 mm NB

(nominal bore), because that is what is readily commercially available. NB and its US

near-equivalent NPS (nominal pipe size) are themselves specifications, rather than sizes.


Design manualsCompanies frequently have in-house design manuals which are a formal way to share

the company’s experience of the processes it most often designs. These manuals are

not in the public domain, because they contain a significant portion of the company’s

know-how. They are jealously guarded commercial secrets.

Some national and international standards are also essentially design manuals—for

example, PD5500 used to be a British Standard and is now more of a design note or

manual which embodies many years of experience of safe design of a safety-critical

piece of equipment (namely the unfired pressure vessel), despite being superseded in

2002 by a European standard (BS EN 13445).

Rules of thumbRules of thumb are a type of heuristic, and are usually very simple calculations which

capture knowledge of what tends to work. As Koen points out, there are only heuris-

tics in engineering.

Rules of thumb do not necessarily replace more rigorous (but still heuristic) analy-

sis when it comes to detailed design, but their condensation of knowledge gained

from experience provides a quick route to the “probably workable” region of the

design space, especially at the conceptual design stage.

Rules of thumb are only ever good in a limited range of circumstances. These

limitations have to be known and adhered to if they are to be valid (though experts

might knowingly break this rule on occasion). Rules of thumb encapsulate experi-

ence, and are therefore better than first principles design.

It should be noted that simulation and modeling are a kind of first principles

design, and cannot be used to generate valid rules of thumb.

It should also be understood that first principles design, generally speaking, doesn’t

work, and that literature examples of successful first principles design are often no

such thing when investigated.

ApproximationsAll is approximation in engineering. If you think you are precisely right, you are

precisely wrong. Engineers who grew up in the era of the slide rule know that

anything after the third significant figure is at best science, rather than engineering.

We need to know how precise and certain we have to be in our answers in order

to know how rigorous to be in our calculation. Very often, in troubleshooting exer-

cises, knowing the usual interrelationships between a few measurements is all that is

needed to spot the most likely source of problems.


Coarse approximations will get us looking in the right general area for our

answers, allowing design to proceed by greater and greater degrees of rigor as it homes

in on the area of plausible design.

Professional judgmentDouglas gives a figure in “Conceptual Design of Chemical Processes” of 105�108

possible variations for a new process plant design, and he leaves out many of the

important variables. This is a far lower number than the possible positions in chess,

but it is similar to the number of patterns memorized by an experienced chess player.

An engineer’s professional judgment allows them to semi-intuitively discern approaches

to problems which might work in a similar way to the experienced chess player. They will

summarily discard many blind alleys which a beginner would waste time exploring, and

include options which beginners would be unlikely to think of. They will know which

simple calculations will allow them to choose quickly between classes of solution.

Consequently, experts can quickly achieve outcomes which less experienced prac-

titioners might never arrive at. This judgment takes many years of practice to develop,

but its development may be started in an academic setting, and this book will attempt

to assist in this task.

STATE OF THE ART AND BEST ENGINEERING PRACTICE

Six blind men who had never seen an elephant were asked to see what they could makeof one which had wandered into their village.

The first man touched its leg, and concluded it was like a pillarThe second man touched its tail and thought it like a ropeThe third who touched its trunk thought it was like the branch of a treeThe fourth felt its ear, and thought it was like a fanThe fifth felt its side, and thought it like a huge wallThe sixth who felt its tusk thought it like a pipeThey began to argue about who was right about the elephant. Each insisted that he was

right.A wise man passing by asked them, “What is the matter?” They explained that they each

thought the elephant was quite different in form.The wise man explained that all of them were correct, which made them happy and

allowed them to stop arguing about who was right.

The man in this story was clearly also wise enough not to point out to the villagers

that, as they were blind, their understanding was rather partial, and none had grasped

what a whole elephant was like to a sighted person, let alone an elephant keeper. No

one thanks you for being that wise.

When I serve as an expert witness, the court requires me to differentiate between

fact (perceptible directly to human senses or detectable with a suitable calibrated


instrument), current consensus opinion (that held by the majority of suitable qualified

professionals), and other opinion. These are held by the court to represent progres-

sively weaker evidence. UK civil courts have a lower standard of proof than our

professional one (they are content with a mere “more likely than not”), but their defi-

nition of sound professional opinion is a good one.

Koen essentially denies the existence of facts in engineering from a rather philo-

sophical point of view, but he defines best engineering practice rather similarly, as cur-

rent consensus professional opinion (essentially that part of a Venn diagram which

would represent the overlap between the heuristics of all professionals).

I agree with these definitions, which means that best engineering practice is always

changing, and no single engineer is an entirely reliable source of best engineering

practice.

My experience as an expert witness has, however, taught me that we have, as pro-

fessionals, an idea of the gap between our personal practice and common practice. We

know whether each of our heuristics is on the fringes or at the heart of consensus/

best practice. Even those maverick professionals who hold fringe ideas which they

think better than best practice know that these opinions are not commonly held.

This means that only current practitioners who regularly engage professionally

with other practitioners are in touch with the moving target of best practice. It also

means that those closest to the heart of a discipline tend to have the greatest concor-

dance between personal heuristics and consensus heuristics. This does not prevent

people far from the heart of the discipline from holding fringe opinions strongly; it

just keeps them from understanding that they are fringe opinions.

THE USE AND ABUSE OF COMPUTERS

Back in 1999 the IChemE’s Computer Aided Process Engineering (CAPE) working

group produced Good Practice Guidelines for the Use of Computers by Chemical

Engineers. These have never been superseded, but they seem to have been largely

forgotten in the interim.

The guidelines emphasize the legal and moral responsibility of the professional

engineer to ensure the quality and plausibility of the inputs to and outputs from the

system, to understand fully the applicability, limitations, and embedded assumptions in

any software used. It also emphasizes the importance of being properly trained to use

the software, and only using fully documented software validated for the particular

application.

They emphasize the primary importance of understanding the problem one is

trying to solve, working within proper engineering limits, of taking into consideration

not just dynamic but transient conditions and, most of all, of applying sensitivity anal-

ysis to the results produced, especially for those areas identified as the most important


to a successful outcome. The severity of the consequences of being wrong should also

be taken into consideration.

They warn users to beware of the assumptions implicit in software, and to know

where default values exist, and to be able to back-trace any data used to a validated

source. This allows the documented accuracy of the data in the range used to be

known, as well as whether it is valid in that range.

They recommend contacting more senior engineers and/or the suppliers of

the software in the event of any uncertainty at all about the correctness, accuracy, or

fitness for purpose of any software or outputs.

In several places, they specifically recommend suspicion about the outputs of

computer programs, and an assumption of guilt until proven innocent.

They say again and again, in different ways, that software cannot be a substitute for

engineering judgment, and its use without understanding is a dangerous abandonment

of professional responsibility.

However, I have seen many times, in both academic circles and in recent gradu-

ates, a failure to understand this simple truth. Computers may be a little faster than

they were in 1999, but they are no closer to being people. All the potential problems

of software are still there, and the awareness of their limitations has decreased. In my

view, this is a disaster waiting to happen.

FURTHER READINGCDIO Initiative: See ,www.cdio.org..Ferguson, E., 1994. Engineering and the Mind’s Eye. MIT Press, Cambridge, MA.IChemE CAPE Working Group, 1999. The Use of Computers by Chemical Engineers: Guidelines for

Practicing Engineers, Engineering Management, Software Developers and Teachers of ChemicalEngineering in the Use of Computer Software in the Design of Process Plant. Institution ofChemical Engineers, London.

Koen, B.V., 2003. Discussion of the Method. Oxford University Press, New York, NY.Meadows, D.H., 2009. Thinking in Systems. Routledge, New York, NY.Pahl, G., Beitz, W., 2006. Engineering Design: A Systematic Approach. Springer, London.Petroski, H., 1992. To Engineer Is Human: The Role of Failure in Successful Design. Vintage, New

York, NY.Wujek, T., Undated. The Marshmallow Challenge. ,http://marshmallowchallenge.com..Vincenti, W.G., 1990. What Engineers Know and How They Know It: Analytical Studies from

Aeronautical History. Johns Hopkins University Press, Baltimore, MD.


http://www.cdio.org

http://refhub.elsevier.com/B978-0-12-800242-1.00001-3/sbref1










http://marshmallowchallenge.com



CHAPTER 2

Stages of Process Plant Design

GENERAL

Process plant design (and indeed almost all design) proceeds by stages which seem not

so much to be conventional as having evolved to fit a niche. The commercial nature

of the process means that minimum resources are expended to get a project to the

next approval point. This results in design being broken into stages leading to three

approval points, namely, feasibility, purchase, and construction.

This is why Pahl and Beitz’s systematized version of the engineering design process

resembles that which applies to all engineering disciplines (including chemical engi-

neering’s process plant design) as practiced by professionals. It may very well also apply

to fashion design. Design is design. Is design.

Note that in recommending Pahl and Beitz’s approach I am not seeking to enter

the academic debate on how the design process ought to be done. Having read many

books on engineering design across many disciplines, I found Pahl and Beitz’s descrip-

tion to be one of the closest to how design is done. That is the subject of this book.

The basically invariant demands of the process are the reason why everyone who

designs something professionally does it basically the same way, even though chemical

engineers are often nowadays explicitly taught a radically different approach in univer-

sity (if they are taught any approach at all).

CONCEPTUAL DESIGN

Conceptual design of process plants is sometimes carried out in an ultimate client

company, more frequently in a contracting organization, and most commonly in an

engineering consultancy.

In this first stage of design, we need to understand and ideally quantify the constraints

under which we will be operating, the sufficiency and quality of design data available, and

produce a number of rough designs based on the most plausibly successful approaches.

I am told that, in the oil and gas industry, the conceptual stage starts from a pack-

age of information known as Basic Engineering Design Data (BEDD), which is often

confused with (Process) Basis of Design. BEDD includes, typically, information to

start the concept design such as:

• General plant description

• Codes and standards


• Location

• Geotechnical data

• Meteorological data

• Seismic design conditions

• Oceanographic design conditions

• Environmental specifications

• Raw material and products specifications

• Utilities

• Flares

• Health Safety Environment (HSE) requirements

Other industries have alternative formats, but initial information packages ideally

cover many of the same areas.

Practicing engineers tend to be conservative, and will only consider a novel pro-

cess if it offers great advantages over well-proven approaches, or if there are no proven

approaches. Reviews of the scientific literature are very rarely part of the design

process. Practicing engineers very rarely have the free access to scientific papers which

academics enjoy, and are highly unlikely in any case to be able to convince their

colleagues to accept a proposal based on a design which has not been tried at full scale

several times, preferably in a very similar application to the one under consideration.

The conceptual stage will identify a number of design cases, describing the outer

limits of the plant’s foreseeable operating conditions. Even at this initial stage, designs

will consider the full expected operating range, or design envelope.

The documents identified in Chapter 3 are produced for the two or three options

most likely to meet the client’s requirements (usually economy and robustness). This

will almost always be done using rules of thumb, since detailed design of a range of

options (the majority of which will be discarded) is uneconomic.

This outline design can be used to generate electrical and civil engineering designs

and prices. These are important, since designs may be optimal in terms of pure “pro-

cess design” issues like yield or energy recovery, but too expensive when the demands

of other disciplines are considered.

At the end of the process, it should be possible to decide rationally which of the

design options is the best candidate to take forward to the next stage. Very rarely, it

will be decided that pilot plant work is required, and economically justifiable, but this

is very much the exception; design normally proceeds to the next stage without any

trial work.

There are academic arguments for including formal process integration studies at

this stage, though this is incredibly rare in practice. The key factor in conceptual stud-

ies is usually to get an understanding of the economic and technical feasibility of a

number of options as quickly and cheaply as possible. As many as 98% of conceptual

designs do not get built, so you don’t want to spend a fortune investigating them.


Client companies have advantages over contractors in carrying out conceptual

designs, as they may have a lot of operating data unavailable to contractors, however,

they do usually lack real design experience.

Contractors are in the opposite situation, while the majority of staff employed by many

consultancies tend to have neither hands-on design experience nor operational knowledge.

In an ideal world, therefore, client companies would collaborate with contractors

to carry out conceptual design. In the real world, this cooperation/information shar-

ing is less than optimal.

“CONCEPTUAL DESIGN OF CHEMICAL PROCESSES”

Douglas wrote a book of this name which essentially attempts to design chemical pro-

cesses (whatever they are), rather than process plants.

He understands that the expert designer proceeds by intuition and analogy, aided

by “back of the envelope” calculations, but sees the need for a method which helps

academics and beginners to cope with all the extra calculations they have to do while

they are waiting to become experts (who know which calculations to do).

The arguments underlying the academic approach which has since been built on

Douglas’s approach are helpfully set out in explicit detail. There is an assumption that

the purpose of conceptual design is to decide on process chemistry and parameters

such as reaction yield. Choices between technologies (the usual aim of conceptual

design exercises) are not considered. Pumps are assumed to be a negligible proportion

of the capital (capex) and running (opex) cost, and heat exchangers are assumed to be

a major proportion of capex and opex.

It is implicit in the chain of assumptions used to create the simplified design meth-

odology that a particular sort of process is being designed. Like all design heuristics,

the methodology has a limited range of applicability. While it mentions other indus-

tries, it is based throughout upon examples taken from the petrochemical industry,

and it is clear that the assumptions it makes are most suited to that industry.

Having declined to consider many items which are of great importance in other

industries, Douglas finds time for pinch analysis, which was quite new when the book

was written. Perhaps this really was a worthwhile exercise for the novice process

designer in the petrochemical industries of the 1980s, but there are many process plant

designs in 2014 which do not have a single heat exchanger.

In the majority of industries, process chemistry is a job for chemists, and from the

plant designer’s point of view is in any case usually limited to choosing between a

number of existing commercially available process technologies.

Douglas offers a plausible approach to the limited problem he sets out to solve,

few of whose assumptions I can argue with in the context of his chosen example.

He attempts to offer the beginner a way to choose between potential process

23Stages of Process Plant Design

chemistries and to specify the performance of certain unit operations in a rather old-

fashioned area of chemical engineering.

However, the problem which this methodology seeks to address is not one I have

ever been asked to find a solution for. When I am asked to offer a conceptual design,

I am being asked to address different questions, on plants with a different balance of

cost of plant components. Petrochemical plants of the sort used as the example in this

book do not really get built in the developed world any more.

The approach does, however, hang together coherently, in a way more recent

developments based on it do not. A good amount of effort goes into constructing as

rigorous a costing as is possible at the early design stage (ignoring the issue of the

items which are left out).

In essence this book seems, in my view, to reflect the slight wrong turns and over-

simplifications which, followed by successive oversimplifications and misunderstand-

ings, led to the utterly unrealistic approaches common in academia nowadays.

The attraction of the approach to academics is presumably that it is intended to

allow people who have never designed a process plant (like the majority of academics)

to use the skills they do have to approximate an early stage design process.

The approach is a tool to allow a very inexperienced designer who does not have

access to expert designers to simplify the design of a certain sort of petrochemical

plant to the point where they can mathematically analyze the desirability of a small

number of parameters such as degrees of reactor yield and energy recovery.

This approach is just another design heuristic, and like all heuristics it has a limited

range of applicability. If the very specific assumptions it makes to achieve this aim are

not met, its use is invalid.

Even if they are met, process integration at conceptual design stage is both uneco-

nomic and unwise, for reasons I discuss later in the book.

Modeling as “conceptual design”Much work thought of as conceptual design, especially in the case of modifications to

existing plant, takes place in large consultancies and operating companies. Because

plant operating companies and consultants lack real whole-plant design experience,

this may feature some elements of the academic approaches derived from Douglas’s

approach discussed in the last section, combining the application of network and

pinch analysis to the output of modeling and simulation programs.

The scope of such studies is usually for a small number of unit operations rather

than a whole plant, and it is by talking solely to those carrying out such studies (and

each other) that academics get the impression that their techniques are used in indus-

try. Academics tend to stay in touch with the kind of students who do well in exams

and who are preferred by such operating companies.


However, this work differs from the design techniques of professional process plant

designers (as I define them in Chapter 1) in a number of ways, most notably the following:

The “conceptually designed” plant will not be built by those carrying out the con-

ceptual design exercise. The contractors who will build it may use this “design” as the

basis of their design process, but since it is they who will be offering the process guar-

antee, they will redesign from the ground up. They will, however, probably not point

this out to the client, who will consequently retain the impression that they designed

the plant.

Genuine information from pilot studies on the actual working plant allows the

model used to be fed with a specific and realistic design envelope, and for its outputs

to be validated on the real plant. This is very different from using the modeling pro-

gram in an unvalidated state.

Thus, such conceptual design studies, while potentially very valuable as a debottle-

necking or optimization exercise, are not a true process plant design exercise. The

modeling, simulation, and network analysis beloved of research academics are of their

greatest utility in this area, due to the availability of the large quantities of specific data

which such approaches require to yield meaningful results.

However, I would still suggest to those carrying out such exercises that they should

be willing to listen to suggestions from contracting companies if they would like to

arrive at a safe, robust, and cost-effective solution. There is no substitute for experience.

All should understand that professional judgment is still superior to the output of

computer programs, however much effort went into the pilot trials and modeling

exercises, but human nature leads people to wish to benefit from costs which have

been irreversibly incurred. If you have spent a great deal of resources on these studies,

it may be hard to accept that you could have simply got a contractor to design the

plant without spending the resources. The sunk cost fallacy needs to be avoided.

FRONT END ENGINEERING DESIGN (FEED)/BASIC DESIGN

If the design gets past the conceptual stage, a more detailed design will be produced,

most commonly in a contracting organization.

Competent designers will not use the static, steady state model used in educational

establishments, but will devise a number of representative scenarios which encompass

the range of combined process conditions which define the outer limits of the design

envelope. All process conditions in real plants are dynamic; they do not operate at

“steady state.” They may approximate the steady state during normal operation if the

control system is good enough, but they must be designed to cope with all reasonably

foreseeable scenarios.

The process engineer would normally commence by setting up (most commonly

in Excel) a process mass and energy balance model linking together all unit operations,


and an associated Process Flow Diagram (PFD) for each of the identified scenarios.

It is usually possible to set up the spreadsheets so that the various scenarios are pro-

duced by small modifications to the base case.

In certain industries (especially the oil and gas sector) a modeling system like Pro

II might be used for this stage, but across all sectors this would be the exception rather

than the rule, because business licenses for such programs are very expensive, and the

available unit operations tend to be tailored to a specific industry sector.

A more accurate version of the deliverables from the previous stage would be pro-

duced, based on this more detailed design/model and, wherever possible, bespoke

design items would be substituted with their closest commercially available alterna-

tives, and the design modified to suit.

Process designers would normally avoid designing unit operations from scratch,

preferring to subcontract out such design to specialists who have the know-how to

supply equipment embodying the specialist’s repeated experience with that unit

operation.

Drawings at this stage should show the actual items proposed, as supplied by

chosen specialist suppliers and subcontractors. Even such seemingly trivial items as the

pipework and flanges selected should be shown on the drawings, as they are supplied

by a particular manufacturer, and pricing should be based on firm quotes from named

suppliers.

The drawings should form the basis of discussions with, at a minimum, civil and

electrical engineering designers, and a firm pricing for civil, electrical, and software

costs should be obtained.

All drawings and calculations produced would be checked and signed off at this stage

by a more experienced—ideally chartered/licensed professional—chemical engineer.

Once that is done, a design review or reviews can be carried out, considering lay-

out, value engineering, safety, and robustness issues. Where necessary, modifications to

the process design to safely give overall best value should be made.

DETAILED DESIGN

Or “Design for Construction.” This virtually always takes place in a contracting orga-

nization. The detailed design will be sent to the construction team, who may wish to

review the design once more with a view to modifying it to reflect their experience

in construction and commissioning.

Many additional detailed subdrawings are now generated to allow detailed control

of the construction of the plant. The process engineer would normally not have

much to do with production of such mechanical installation drawings, other than par-

ticipation in any design reviews or Hazard and Operability Studies (HAZOPs) which

are carried out.


The junior process engineer will, however, have much to do with producing

documents such as the datasheets, valve, drive, and other equipment schedules, instal-

lation and commissioning schedules, and project program.

This painstaking work is required to allow the procurement of the items which are

described in them by nonengineers. It is not so much design work as contract

documentation.

SITE REDESIGN

This does not usually feature much in textbooks on the subject, but it is not uncom-

mon for designs to pass through all the previous stages of scrutiny and still be missing

many items required for commissioning or subsequent operation.

It is a lot cheaper to move a line on a drawing than it is to reroute a process line

on site. If communication from site to designers is managed very poorly in a company,

expensive site modifications may be required on many projects before the problem

comes to the attention of management.

Commissioning and site engineers are rarely involved in the design process

(though they should be involved in the HAZOP) but often find these omissions when

they review the design they have been given, or worse still, find the error on the plant

after it was built. This is an expensive stage at which to modify a design, but in the

absence of perfect communication from site to design office, it will continue to be

needed.

On behalf of commissioning engineers everywhere, therefore, I would like to

encourage designers to make sure that the following items are included in designs at

the earliest stage:

• Tank drains which will empty a tank under gravity in less than one hour, and

somewhere for the drained content to go to safely,

• Tank vents or vent/vac valves (with protection where required) to allow air to

enter and exit a tank safely,

• Suitable sample points in the line after each unit operation (there may be far more

to this than a tee with a manual valve on it),

• Service systems adequately sized for commissioning, maintenance, and turnaround

conditions,

• Connection points for the temporary equipment required to bring the plant to

steady state operation from cold need to be provided, especially where process

integration has been undertaken,

• Water and air pollution control measures required under commissioning

conditions,

• Access and lifting equipment required under commissioning, maintenance, and

turnaround conditions.


Designers should in my opinion also avoid the following features, which are likely

to be removed by commissioning engineers or operators:

• Permanent pump suction strainers or filters, especially fine mesh filters.

There is a lot more on this in Chapter 17.

POSTHANDOVER REDESIGN

The commissioning engineer may have tweaked or even redesigned the plant to make

it easier for it to pass the performance trials which are used to judge the success of the

design, but the nature of the design process means that while the unit operations have

been tuned to work together, they actually have different maximum capacities.

When the spare capacity in the system is analyzed, it is usually the case that the

output of the entire process is limited by the capacity of the unit operation with the

smallest capacity. This is a restriction of capacity or “bottleneck.” Uprating this rate-

limiting step (or a number of them) can lead to an economical increase in plant

capacity.

Similarly, it might be that services which were slightly overdesigned to ensure that

the plant would work under all foreseeable circumstances, and optimized for lowest

capital cost rather than lowest running cost, can be integrated with each other in such

a way as to minimize cost per unit of product. This is important, because prices for a

plant’s product tend to fall over its operating life, as competing plants which are built

in low-cost economies or based on newer, better processes bring prices down.

It should be noted that those carrying out this kind of operation have at their dis-

posal a lot more data than whole-process designers. Clever mathematical tools were

developed back in the 1970s to facilitate the energy integration process, and their

conceptual approach has since been applied to mass flows, including those of water

and hydrogen.

These were devised to be optimization tools, rather than process design tools. As I

discuss elsewhere in this book, there are a number of inevitable drawbacks to their

misuse as design tools, which are more serious the earlier they are used in the design

process.

This book does not have much to say about post-handover design activity, as it

does not meet a number of my criteria to qualify as process plant design. Most nota-

bly, it does not involve designing a whole plant; there is a great deal of site-specific

detailed information available to validate computer models; and small improvements

in performance are within the resolution of the design process.

This activity is, to my mind, aftermarket tuning of a plant which has been

designed by others, rather than designing one from scratch, the subject of this book.


UNSTAGED DESIGN

Sometimes it is more important to get a design completed quickly than to spend time

optimizing cost, safety, and robustness. These things still matter, and have to be

addressed to at least a minimum standard, but there can be commercial drivers which

mean that getting the product of the process on the market as soon as possible is the

most important factor.

The premier example of this would be design of systems producing a product

which is subject to patent, especially pharmaceuticals. Once patent protection is

removed, generic substitutes manufactured in low-cost/wage economies will usually

rapidly out-compete the patented version.

Pretty much all the money which will be made on the product will come in

between the day it hits the shelves and the day the patent expires. Thus, getting the

product to market quickly is more important than plant design optimization, to the

dismay of my colleagues who work in pharma.

PRODUCT ENGINEERING

European chemical engineering bodies such as FEANI are encouraging the promotion

of “product engineering” into chemical engineering curricula. It should, however, be

noted that “product engineering” is not the same thing as “product design,” a term

used in academia for something which is often not any kind of engineering.

Product engineering is the name given to shortening the classic approach

described in this chapter and running it alongside a product design program. The deli-

verables of product engineering are identical to those listed earlier in this chapter.

Price, practicality, and safety issues need to be as much at the forefront of the process

as they are with the classic approach.

Process Plant Designers design coordinated assemblies of machines (gubbins) to

make chemicals (stuff). Our plants make stuff, rather than gubbins, and our plants are

made of gubbins rather than stuff. Mechanical engineers design gubbins, and chemists

work with stuff.

Academic “product design” is often just rebranded chemistry content, with little

consideration of plant or production cost, safety, or robustness. I have even seen it

used to describe an exercise in which students were asked to research and design a

university teaching module.

The design methodology of professional product design is just like that of the pro-

fessional plant designer. Pugh’s “Total Design” is about product design, but is far

closer to a description of how process plant design is actually undertaken than almost

all books by “chemical engineers.”


FAST-TRACKING

Mixing the natural stages of design in order to accelerate a program is a reasonably

common approach, which may be used in professional practice (though it has a down-

side). It is telling that contractors who are asked to make a program move faster (or

“crash the critical path”) are usually given acceleration costs to compensate them for

the inherent inefficiency of the fast-tracked process.

As well as costing more for the reasons given below, fast-tracking is commonly

held to always increase speed at the cost of quality, whether that be design optimiza-

tion quality and/or quality of design documentation.

The standard approach practically minimizes the amount of abortive work under-

taken, since each stage proceeds on the basis of an established design envelope and

approach. Each stage refines the output of the preceding stage, requires more effort,

and comes to a more rigorous set of conclusions than the preceding stage.

When stages are mixed, the more rigorous steps are carried out earlier in the pro-

gram when the design has a larger number of variables. We may well therefore need

to have a larger team of more experienced people working on the project. Even with

the improved feel for engineering given by using more experienced engineers, it is

much more likely in fast-tracking that a design developed to quite a late stage will

need to be binned, and the process restarted from the beginning of the blind alley

which the design went down. Of course, if you are in a sector where you are making

an extremely profitable product, this will be less of a concern than if you are in a less

lucrative field.

Generally, costs for product by sector may be ranked as follows:

BiopharmaceuticalsPharmaceuticals/nanotech products and the likeFine chemicalsOil and gasBulk chemicalsWater and environmental

Oil and gas have an anomalous position in this hierarchy (and are highly profitable)

because they produce huge volumes of a product whose price is effectively set largely by an

international cartel.

Conceptual/FEED fast-trackingIn a contracting company or design house, conceptual design can be very quick

indeed. Senior process engineers know from experience which is likely to be the best

approach to an engineering problem. A few man-hours may be all that they require

to rough something up.


However, this means that a client who wishes to skip the stage where they or a

consultant do the initial designs is ceding control of the design to a contractor. This is

not without a downside—senior process engineers in contracting organizations will

almost certainly come up with a design which will be safe, cost�effective, and robust,

but a design based entirely on generic experience is unlikely to be the most innovative

practical approach.

FEED/detailed design fast-trackingIf a project is definitely going to go ahead, and the contractor has been appointed,

FEED and detailed design can be combined, and the operating company/ultimate cli-

ent can take part in the design process. This was quite a common approach using the

Institution of Chemical Engineers (IChemE) “Green Book” contract conditions back

at the start of my career.

This approach can produce a very good quality plant, which is unusually fit for pur-

pose, but the FEED study is used as the basis of competitive tendering. The removal of

a discrete FEED study, along with that of the usually adversarial approach between cli-

ent and contractor in process plant procurement, does seem to inflate costs somewhat.

Design/procurement fast-trackingTime is generally lost from the originally planned design program at each stage of

design such that the later stages—which produce the greatest number of documents,

employ the most resources, and are most crucial to get right—often proceed, unhelp-

fully, under the greatest time pressure.

There are barriers to communication between the various stages of design which

need to be well managed if conceptual design is to lead naturally to detailed design and

from there to design for construction. If the communication process is managed poorly,

unneeded redesign may be carried out by “detailed” or “for construction” design teams

who do not understand the assumptions and philosophy underlying the previous design

stage. Alternatively designs may have to be extensively modified on site during con-

struction and commissioning stages, usually at the expense of the contractor.

Design/procurement fast-tracking is quite popular as a response to time lost in ear-

lier stages for projects where the end date cannot move. As soon as an item’s design

has been fixed, the procurement process is started, especially with long lead time items

like large compressors.

The nature of design being what it is, this can result in variations to specifications

for equipment design after procurement has started, and this usually carries a cost pen-

alty, as does the high peak manpower loading, duplication of designs, and backing-out

of design blind alleys inherent in this approach.


The fast-track to bad designThe “chemical process design” approach popular in academia attempts to mix all

design stages prior to construction, and often substitutes modeling for construction

(a mistake I comment on elsewhere in the book). It is frequently argued by those

who teach this that it is how design ought to be done, as sustainability concerns mean

that we need to approach thermodynamic limits to energy recovery.

The first problem I have with this is that sustainability is a highly politicized term.

A Greenpeace member, a trade union activist, and a Chartered Chemical Engineer

might use the term to mean three completely different things. The IChemE have

helpfully written guidelines on what it means to chemical engineers, in the form of

sustainability metrics. We engineers like a metric, so that we can analyze a problem

and its possible solutions at least semi-quantitatively.

The IChemE interpretation does not support shooting for theoretical perfection in

a small number of aspects of process design. Chemical engineers are concerned about

the environment, but we know that the curves of process yield, energy recovery,

safety, and environmental protection against cost are exponential. Both perfect

processes and complete safety are infinitely costly.

My second problem is that it is not just that you cannot optimize a process before

you design it in detail: you cannot really optimize a process before you have built it.

At the conceptual design stage the design subcomponents are not items you can actu-

ally buy economically. There are a number of reasons for this which are as follows:

Firstly, sizes of available process equipment are not a continuous variable. There

are minimum and maximum available sizes, and size increases in discrete steps. This is

true of the very simplest items such as lengths of pipe. You can have special items

made to order, but they are far more expensive than stock items, take far longer to

deliver, and are more likely to have unforeseen problems.

Secondly, no plant is ever built exactly as specified. Sometimes this is due to errors

in construction or poor QA in materials or construction but, more significantly, many

plants need site-level redesign due to design errors, unforeseen consequences, or a late

change in client specification.

Thirdly, this approach takes no consideration of the interactions with other disci-

plines (most notably civil and electrical engineering) of process design choices. These

can be very significant, far more significant than the cost implications of theoretically

tweaking a virtual plant for small increases in energy efficiency or yield.

Fourthly, it is commonplace in this approach to take no real consideration at all of

the impact of process choices on either capital or running costs of the plant. If you

aren’t costing, you aren’t engineering, but those who practice “chemical process

design” apparently consider cost only by comparing the cost of feedstock with that of

product. If product can be sold for more than feedstock can be bought, the process is


deemed to be economic. Thus the marginal cost of yet another heat exchanger is set

to zero, such that it is always viable to go one step closer to theoretical perfection.

I have been told that the application of aspects of this approach to real plant design

has led to processes whose safety factors are so tight that it is thought unwise by expe-

rienced engineers to attempt debottlenecking, though management still expects it.

This is arguably what the approach really does: any savings which might be found

will come out of safety factors and plant operability. It all seems very reminiscent to

me of the erosion of safety factors which presage disaster in many of Henry Petroski’s

examples. It all seems terribly clever until your bridge falls down, or your plant is

replaced by a crater.

All of this said, there are circumstances where simulation can be used to partially

replace design: if you have a great deal of operating data for exactly the plant you are

“designing,” you can tune and validate the model, such that you are no longer design-

ing from first principles and generic data, but have an empirically verified simulation

of the actual plant.

This is the case if you are working for an operating company or a contractor/operator

with access to such data, and the spare time necessary to tweak the simulation. However,

you may not necessarily understand why the simulation behaves as it does, even in the

case where you seem to have made it behave exactly like the real plant.

If you do not understand your model, it is worse than useless.

FURTHER READINGAzapagic, A., 2002. Sustainable Development Progress Metrics Recommended for Use in the Process

Industries. Institution of Chemical Engineers, London.Pahl, G., Beitz, W., 2006. Engineering Design: A Systematic Approach. Springer, London.Petroski, H., 2012. To Forgive Design. Harvard University Press, Cambridge, MA.Pugh, S., 1990. Total Design. Prentice Hall, Upper Saddle River, NJ.







CHAPTER 3

Process Plant Design DeliverablesOVERVIEW

“Deliverables” may not be the most elegant word, but it is freighted with meaning. It

comes from project management, and it reminds us that the immediate purpose of the

design process is to deliver to a client a set of documents which they can use to build

a plant, or more usually approve formally, so that the designer’s company can build it.

Less frequently, the plant itself is described as a deliverable, but we will restrict our-

selves in this section to the drawings and other documents which are commonly used

to transmit design and construction intent from the designer to the construction team.

The following sections list these documents in the rough order in which they are first

produced, although revision of such documents may be ongoing throughout the project.

These are only the most important and commonplace deliverables; I am deliber-

ately omitting many sector-specific deliverables.

DESIGN BASIS AND PHILOSOPHIES

The output from the conceptual design stage may sometimes be restricted to guidance

on the approach which should be followed in subsequent design stages: a design basis

or design philosophy.

These terms are sometimes taken to be the same thing, but I will differentiate

between them as follows:

In professional practice, a design basis will usually be a succinct (no more than a

couple of sides of A4) written document which might define the broad limits of the

Front End Engineering Design (FEED) study, including such things as operating and

environmental conditions, feedstock and product qualities, and the acceptable range of

technologies.

Design Philosophies by contrast may run to 40 pages, including overpressure protec-

tion philosophies, vent, flare, and blowdown philosophies, isolation philosophies, etc.

Clients often specify a design philosophy in their documentation, and individual

designers and companies may have their own in-house approaches. It is good practice

for a formal design philosophy to be written as one of the first documents on a design

project. Similarly a safety and loss prevention philosophy is ideally produced early on

in the design process.


The design philosophy should record the standards and philosophies used, together

with underlying assumptions and justifications for the choice. This is both to allow a

basis for checking in the detailed design stage and for legal purposes.

In the absence of written philosophies, a second engineer at detailed design stage

might attempt to apply the ones he or she would have chosen, and the plant may

become subject to pointless expensive and extensive redesign.

SPECIFICATION

There are a number of types of specifications which are produced or introduced at

various stages of the design process. We might split them initially into specific and

boilerplate categories. Let us first dispense with the second category.

Boilerplate is a term used in legal contexts, but it originates in engineering. A

rating-plate containing standard text was required to be attached to a boiler by the

Boiler Explosions Act (1882), and the term subsequently came to mean standard text

which is cut and pasted into documents.

Much of this sort of text has been described as “write only” documentation

(or WOD—we engineers love a three-letter acronym (TLA)), because someone has to

write it but virtually no one has to read it. The consultant draws the attention of the

designer to it, and they in turn draw it to the attention of their suppliers.

The tender documentation sent out to contractors by consultants frequently contains

great volumes of boilerplate specification, lists of applicable (or potentially inapplicable

if your consultant is lazy and/or risk averse) legislation and standards, and references to

all the other things which the conceptual designers didn’t personally consider, but think

someone else should. I am not convinced that this frequently lazy approach actually

provides the degree of legal protection those responsible for it imagine.

More usefully, there will be a far thinner volume of specifications which inform

the definition of the design envelope. The expected quantities and qualities of feeds

into the process should be included, as well as a description of product quality and

quantity. These descriptions will ideally be in the form of ranges of concentrations,

flows, temperatures, pressures, and so on. There may be statistical information to

allow the designer to understand the distribution of likelihood of various conditions.

There may be reference to specifically applicable standards, legislation and so on.

This differs from boilerplate as those responsible for the previous stage of design have

identified that these documents are likely to really matter to this specific design.

The separation of these useful specifications from the boilerplate is often the first

job of the contractor’s plant design engineers. The boilerplate has to be checked for

anomalous content alongside the real design process, but that is not usually on the

critical path. It usually suffices to send out (largely unread) relevant sections to those

offering prices for the equipment to be purchased.


PROCESS FLOW DIAGRAM (PFD)

The Process Flow Diagram (PFD) is a visual representation of the mass and energy

balance. The PFD treats unit operations more simply than the Piping and

Instrumentation Diagram (P&ID—see next section). Unit operations are shown using

British Standard (BS) P&ID symbols or sometimes as simple blocks, pumps are shown, as

are main instruments (Figure 3.1).

The lines on this diagram are labeled in such a way as to summarize the mass and

energy balance, with flows, temperatures, and compositions of streams.

Please do not call this a flow sheet, as this term is used to mean quite a few differ-

ent things (for my suggested use of the term in a process plant design context see

Chapter 14). Neither should you think that a simulation program printout is a substi-

tute for a professional PFD conforming to a recognized standard.

The Block Flow Diagram (BFD) used in academia as a simplified substitute for a

PFD is not something I have seen used in practice, other than when drawn on a beer-

mat in a pub discussion.

The general British Standard for engineering drawings, BS 5070 applies to the

PFD, as well as BS EN ISO 10628. The symbols used on the PFD should ideally be

taken from BS EN ISO 10628, BS1646, and BS1553.

Figure 3.1 PFD for the pH correction section of a water treatment plant.

37Process Plant Design Deliverables

PIPING AND INSTRUMENTATION DIAGRAM

The P&ID is a drawing which shows all instrumentation, unit operations, valves, pro-

cess piping (connections, size, and materials), flow direction, and line size changes

both symbolically and topographically (Figure 3.2). Thus it is not a scale drawing, and

the lines on a P&ID turning corners mean nothing, though the joining of three or

more streams is meaningful.

The P&ID is the process engineer’s signature document, and its purpose is to

show the physical and logical flows and interconnections of the proposed system.

Recording them visually on the P&ID allows them to be discussed with software

engineers, as well as other process engineers. There are a great many variants in addi-

tional features between industries, companies, and countries, but producing the

drawing to a recognized standard makes it an unambiguous record of design intent, as

well as a design development tool. This is, however, rather idealist. I have only ever

worked for one client who produced unmodified BS P&IDs.

The standards for the symbols which should ideally be used by British engineers are

BS EN ISO 10628, BS1646, and BS1553 and, like all engineering drawings, it should

be compliant with BS 5070. Having said how things should be, how things actually are

is that many companies and industries have their own internal standards for P&IDs.

Professional engineers get used to the range of symbols and conventions commonly

used on P&IDs (though I try to shield my students from this at first to avoid confusion).

There are also a number of P&ID conventions which do not appear in standards:

• Flow comes in on the top left of the drawing, and goes out at bottom right.

• Process lines are straight and either horizontal or vertical.

• Flow direction is marked on lines with an arrow.

• Flow proceeds ideally from left to right, and pumps, etc. are also shown with flow

running left to right.

Figure 3.2 Extract from a P&ID.


• Sizes of symbols bear some relation to their physical sizes: valves are smaller than

pumps, which are smaller than vessels, and the drawn sizes of symbols reflect this.

• Unit operations are tagged and labeled.

• Symbols are shown correctly orientated: vertical vessels are shown as vertical, etc.

• Entries and exits to tanks connect to the correct part of the symbol—top entries at

the top of the symbol, etc.

Less complex P&IDs produced during earlier design stages will normally come on a

small number of ISO A1 or A0 drawings, but the P&IDs for a complex plant may be

printed in the form of a number of bound volumes where every page carries a small P&ID

section.

Every process line on the drawing should be tagged in such a way that its size,

material of construction, and contents can be identified thus:

Number showing NB in millimeters—Letter code for material of construction—

Unique line number—Letter code for contents e.g. a line tagged 150ABS004CA would

be a 150 mm NB line made of ABS (plastic), numbered 4, containing compressed air.

Personally, in common with many other designers, I number the main process line com-

ponents first, increasing from plant inlet to outlet. So line 100ABS001CA would, for

example, normally be upstream of line 150ABS004CA above. Design development can,

however, mean that this gets a bit muddled on the as-built version of the drawing.

Every valve and unit operation on the P&ID will also be tagged with a unique

code; a common key is given in Table 3.1:

Table 3.1 P&ID Tag TableValves

MV Manual valve

AV Actuated valveFCV Flow control valve

CV Control valve

ESV Emergency shutdown valve

Unit operations

U Unit

T Tank

P PumpB Blower

C Compressor


The letter code will be followed by a unique number for that coded item.

Similarly, every instrument will be given a code as set out in BS1646 as

follows:

First Letter—measured parameter:

L5 Level

P5Pressure

T5Temperature

Additional letters—what is done with the measurement (you can have more

than one of these):

I5 Indicator

T5Transmitter

C5Controller

The letter code will be followed by a unique number for that coded item, for

example, PIT1 would normally be the first pressure indicator/transmitter on the main

process line.

The British Standards cover these conventions in more detail.

The P&ID is a master document for Hazard and Operability (HAZOP) studies. It

also frequently shows useful termination points between vendor and main contractor,

and between main contractor and equipment supplier.

FUNCTIONAL DESIGN SPECIFICATION (FDS)

An FDS is sometimes called a control philosophy, although both of these terms are

used in other contexts to mean other things. The document I am referring to

describes (ultimately, in practice, for the benefit of the software author) what the pro-

cess engineer wants the control system to do.

It starts with an overview of the purpose of the plant and proceeds to document,

one control loop at a time, how the system should respond to various instrument

states, including failure states.

This is done in clear and straightforward language, designed to be entirely unam-

biguous and comprehensible by nonspecialists.

It is read in conjunction with the P&ID and refers to P&ID components by tag

number, and is used alongside the P&ID in HAZOP studies.

PLOT PLAN/GENERAL ARRANGEMENT/LAYOUT DRAWING

The General Arrangement (GA) drawing shows the plant and pipework as it is

intended to be installed (or as it was installed in the case of an “as-built” GA).


In professional practice a specialist piping or mechanical engineer may produce the

finished drawing, but chemical engineers lay out equipment in space, and produce

this drawing as part of their design process (Figure 3.3).

Drawings should conform to BS5070 and show (as a minimum), to scale, a plan

and elevation of all mechanical equipment, pipework, and valves which form part of

the design, laid out in space as intended by the designer. Where possible, the tag

numbers used in the P&ID should be marked on to their corresponding items on the

GA as well, to allow cross-referencing.

The inclusion of key electrical and civil engineering details is normal in profes-

sional versions, and there are also usually detailed versions for each discipline which

refer back to a common master GA.

Ideally, the drawing will be produced to a commonly used scale (1:100 being

the commonest scale for these drawings), and would be marked with weights of

main plant items. Fractional or odd-numbered scale factors should be avoided.

Sectional views demonstrating important design features are a desirable optional

extra.

Figure 3.3 Section of plot plan/general arrangement/layout drawing.


PROGRAM

A project “program” or “schedule” most usually refers to a Gantt chart which sets out

the planned timescale and resourcing of a project. While a specialist may produce this

in a larger company, chemical engineers should be able to produce a competent pro-

gram as part of the plant design process. In the absence of a resourced program

(Figure 3.4), any estimate of capital cost must be treated with great suspicion.

The overall project is broken down into discrete tasks, and the time and resources

necessary to achieve each of these tasks is estimated. “Milestones” usually appear at

the end of phases or tasks, and are often associated with the production of deliverables,

triggering payment or another phase of the project. “Dependencies” have to be iden-

tified—certain tasks must precede or follow others. Additional time (“float” or

“bunce”) should be added to the minimum reasonable times for each task, to reflect

the uncertainty of the estimate.

A program can then be generated which shows a reasonable estimate of the time

to complete all tasks, which can be analyzed to see which activities set overall project

time. The critical path through a program involves the chain of activities whose com-

pletion is critical to overall program length.

Microsoft market a programming tool called “Project” which can be used to produce

this document to a professional looking standard, though in practice, other specialist tools

such as “Primavera” are at least as commonly used by specialist project programmers.

COST ESTIMATE

Academic approachCost estimation is usually taught in academia using the Main Plant Items/Factorial

method, a method I have never once seen used in professional practice, though the

prices it produces are normally acceptable as very broad ballpark estimates.

Figure 3.4 Example Gantt chart.


It is well explained in Sinnot and Towler (aka Coulson and Richardson 6), a book

all chemical engineers should be familiar with, so I will not reproduce it in detail. In

outline, however, student-style pricing goes like this:

• Price up your main unit operations, probably using curves of unit price against duty.

• Use factors which accompany curves to account for quality of materials, pressure, etc.

• Multiply prices by the CPI (Chemical Price Index) or similar to reflect sector spe-

cific inflation since curves were drawn.

• Convert prices obtained to desired currency at today’s prices.

• Add together all the prices.

• Multiply this total by Lang factors to estimate cost of all the other items and ser-

vices necessary to get a complete plant (around 4).

• Marvel at how big the numbers are (even though they are probably a radical

underestimate).

The technique does, however, raise a number of important issues, and gives a feel

for the relationships between the prices of the inputs to a project.

Students come away with the useful impression that the cost of a complete plant is

a great deal more than the cost of delivering its main unit operations to site, and that

electrical and civil costs in particular are very significant. It also introduces students to

the idea that plants are built to make a profit—one of the Lang factors even has

that name.

It is, however, usually insufficiently powerful to genuinely resolve differences

between options—the margin of error is probably more like 6 50% than the 6 10%

many of my students think it is.

This is, however, incredibly rigorous in contrast to the version of the “Economic

Potential” technique I have seen used by academics, who just want to get to the pinch

analysis as soon as possible, and don’t wish trivia like safety, cost, and robustness to get

in the way.

Their technique is as follows:

• Google the bulk price of the proposed feedstock (F) and expected product (P).

• If P. F, any process which turns F into P is economic.

The former technique might be a bit woolly, but the latter is operating at an accu-

racy of 6 several hundred percent. It is not merely worthless; its use encourages

overly complex and uneconomic design choices. It does, however, have the advantage

of taking only a few seconds.


Professional budget pricingEven the least rigorous real-world pricing exercises tend to start from quoted prices

for main plant items. If you work in a contracting company, reasonably recent quotes

for reasonably similar equipment will usually be available.

Costs of control panels, software, electrical and mechanical installation, civil and

building works will also be priced based upon past quotations or rules of thumb.

Internal cost will be estimated based on experience and/or rules of thumb. A

good chunk of contingency will be added to reflect the high degree of uncertainty at

this early stage of the job.

Someone who does this for a living will be able to produce a 6 30% budget price

in this way in a few hours.

Professional firm pricingIf a company is going to contract to build a plant for a fixed sum of money, it needs

to be certain that it can make a profit at the quoted price.

Equipment prices are obtained from multiple sources for specific items whose spe-

cifications come from reasonably detailed design.

Civil, electrical, and mechanical equipment suppliers and installation contractors

are also invited to tender for their part of the contract.

Internal quotations are also usually obtained from discipline heads within the

company for the internal costs of project management, commissioning, and detailed

design.

There may well be negotiations with all of these sources of information.

Ideally there will be multiple options for equipment supply and construction. A

price based on a single quotation is far less robust than one which has a broader

base.

Once there are prices for all parts and labor, residual risks are priced in. The

profit, insurances, process guarantees, defect liability periods, and so on are then

added.

This exercise can take a good-sized team of people weeks or months to complete,

and the product is a 61�5% cost estimate.

There was a recent article in The Chemical Engineer magazine about a company

using an Excel spreadsheet they had developed to produce Class 3 budget price esti-

mates at an early stage by a process they call “conceptual design emulation.” I have no

idea how well this works, but do I know it isn’t free, although the conventional

approach is far from cheap.


EQUIPMENT LIST/SCHEDULE

A schedule or table of all the equipment which makes up the plant is usually first pro-

duced at FEED stage. Tag numbers from drawings are used as unique identifiers, and

a description of each item accompanies them. There may be cross-referencing to

P&IDs, datasheets, or other schedules.

Similar schedules are produced for all instrumentation, electrical drives, valves, and

lines (Figure 3.5).

Some modern software promises to remove the necessarily onerous task of producing

these schedules from the junior engineer’s tasklist. The nit-picking rigor required is cer-

tainly arguably more suited to the infinitely patient stupidity of computers than the inven-

tive mind of a professional engineer, but schedules are still mostly generated by unlucky

people.

Figure 3.5 Extract from an equipment schedule.


DATASHEETS

Datasheets gather together all the pertinent information for an item of equipment,

mainly so that nontechnical staff can purchase it. Process operating conditions, materi-

als of construction, duty points, and so on are brought together into this document to

explain to a vendor what is required (Figure 3.6).

Figure 3.6 Example of an equipment datasheet.


Data sheets need to be cross-checked with a number of drawings, calculations, and

schedules, and care has to be taken to ensure that they are in accordance with the lat-

est revisions. This is a more skilled task than the generation of schedules, and will

therefore be likely to remain in the purview of young engineers for years to come.

SAFETY DOCUMENTATION

HAZOP studyAs I and all professional engineers use the term, a HAZOP study is a “what-if ” safety

study. It requires as a minimum a P&ID, FDS, process design calculations, and infor-

mation on the specification of unit operations, pumps, etc. as well as probably eight

professional engineers from a number of disciplines.

In an academic setting, the calculations and specifications of equipment may stand

in for the datasheets which would be available to a real HAZOP.

The report produced by the participants will usually nowadays include a full

description of the line-by-line (or node-by-node) permutation of keywords and prop-

erties used in carrying out such a study, but it was more usual in the past to produce a

summary document listing only those items which were identified by HAZOP as

being problematic, what the problems were and how it was intended that they be

avoided.

In today’s litigious environment, a full and permanent record of all that was dis-

cussed and considered in a HAZOP is increasingly considered prudent. This may

most conveniently be achieved by video recording of the entire procedure. Recording

the entire procedure one way or another is now considered best practice.

As students frequently have a great deal of difficulty imagining what they might do

about any problematic upset conditions they have identified, I have included at

Appendix 2 an upset conditions table from “Process Plant Design and Operation—

Guidance to Safe Practice,” by Scott & Crawley which offers useful guidance.

Zoning study/hazardous area classificationZoning the plant with respect to the potential for explosive atmospheres is not a

strictly quantitative exercise (Figure 3.7).


It is common for a small number of engineers to get together with design drawings

to produce a zoning drawing or drawings showing the explosive atmosphere zoning

they think appropriate for the various parts of the plant. There are more details of this

in the chapter on layout.

DESIGN CALCULATIONS

These are usually not really a deliverable at all, since only other process engineers can

understand them. Design proceeds by a number of stages, from initial coarse approxi-

mations to the level of fineness specified in the design brief issued. Academic exercises

are normally carried out around the level of detail used for budget costing (though

they often use tools which would not be used at this stage in practice).

If the design is not utterly novel, heuristic design is realistic for the majority of

items. First principles design should not be favored for items which are commer-

cially available, because this is unrealistic, is the opposite of professional practice

(and, in a teaching environment, it is difficult to detect cheating). If it is desired to

evaluate students’ ability at first principles design, a genuinely completely novel item

should be chosen.

Process designs normally aim to determine certain key dimensions, areas, and

volumes based on a number of parameters. For nonnovel processes there may be

rough rules of thumb, established design guides, standards, or codes of practice.

Figure 3.7 Zoning study/hazardous area classification. Copyright image reproduced courtesy ofDoosan Enpure Ltd.


In professional practice these will be combined to inform a design sizing. Only in the

near-complete absence of relevant guidance of this sort will first principles design be

used, and large design margins will be added to reflect the high degree of uncertainty.

More generally, an understanding of the degree of applicability of the design meth-

ods used and uncertainly around design data should be reflected in a stated added

design margin.

The design calculations should include consideration of construction, materials

selection, cost, and practicality, and should cover all foreseeable operating conditions,

including start-up, shutdown, and maintenance, as well as environmental considera-

tions such as power outage, plausible natural and man-made disasters. The range of

variability in feed stock flows and compositions under normal conditions should be

considered as well as these more extreme variations from steady state.

Opaque outputs from modeling and simulation programs are no substitute for

transparent process design calculations, as it is easily likely that neither the author nor

the checker can readily understand what is going on in the simulation.

In assessing process design calculations it should be borne in mind that the point

of such calculations is not to demonstrate proficiency in mathematics or chemistry.

The point of process design calculations is to produce a minimum specification for an

item, so that the next size up (or sometimes down) commercially available unit can be

purchased with a reasonable degree of confidence in its robustness.

Process design is not a matter of finding exactly the right size item, but of finding

one large and flexible enough such that one can be sure that the commissioning engi-

neer can make it work.

Complementary with the process design calculations are the calculations used to

size pumps, pipework, channels, and so on, which we might collectively refer to as

hydraulic calculations.

Precise determination of dynamic heads by fluid mechanics is extremely difficult

in practice and, furthermore, completely unnecessary. There are a number of heuris-

tics which may be used to carry out rapid determinations of approximate headlosses

to distinguish between competing conceptual designs, and produce initial layouts. The

use of appropriate tabulated values and nomograms for this purpose should be entirely

acceptable for early stage design.

More detailed calculations should be required for the final selected design. These

should at a minimum degree of rigor be based on one of the friction factor methods,

with k-values for fixtures and fittings. All dimensions used in the calculations should

refer back to the dimensioned GA drawing or “iso”. The calculations should be based

on actual selected commercially available pipework, valves, pumps, instrumentation,

and so on. (For those teaching this subject, the internet means that our students will

no longer need to bother manufacturers to obtain this data, so we have no reason not

to require this degree of realism.)


A fairly common approach (and my personal preference) is to use one tab of a

spreadsheet program for each unit operation, and to link the inputs and outputs to the

tabs in such a way that the whole spreadsheet is a standalone model encompassing

mass and energy balance, unit operation sizing, and hydraulic calculations. I use a

standard template which looks like an engineer’s calculation pad. Each tab has a verti-

cal column of these virtual pages in which the argument and calculations for that unit

operation is set down in a logical and readable form (Figure 3.8).

However it is achieved, transparency and clarity of design intent should be the

important factor in evaluating process and hydraulic design calculations. Calculations

should be double-spaced, ideally on an engineers’ calculation pad or electronic facsim-

ile, and every step should refer to any drawings, design standards or other references

upon which it relies.

Figure 3.8 Example intake pipeline design calculations.


ISOMETRIC PIPING DRAWINGS

At the detailed design stage, piping isometrics are produced for larger pipework,

either by hand on “iso pads” or by computer-aided design (CAD) (Figure 3.9):

Isometric piping drawings are not scale drawings, so they are dimensioned. They

are not realistic, pipes are single lines, and symbols are used to represent pipe fittings,

valves, pipe gradients, welds, etc. Lines, valves, etc. are tagged with the same codes

used on the P&ID and GA. Process conditions like temperature, pressure, and so on

may also be put on the iso.

It may well be that “clashes” where more than one pipe or piece of equipment

occupy the same space are only identified at the stage of production of isos, so design

cannot be considered complete before isos are produced.

The purpose of the iso is to facilitate shop fabrication and/or site construction.

They are also used for costing exercises and stress analysis, as they conveniently carry

all the necessary information on a single drawing.

Producing isos by hand is quite time-consuming. There are some CAD systems

now which can automatically produce isometric drawings from the GA drawings.

It is claimed that these reduce drafting errors and inconsistencies, spot clashes ear-

lier, facilitate links to other software such as costing programs, and save time. Hand

drafting is, however, still the norm in many industries.

Figure 3.9 Isometric pipeline drawing.


SIMULATOR OUTPUT

This is not really a deliverable at all, as at best only other process engineers can under-

stand it. The purpose of modeling and simulation on those few occasions where it

might be used by process designers is to provide supporting information.

I have also seen simulator outputs used to attempt to resolve conflicts between

engineers as to which heuristics are most applicable, but in such circumstances it

becomes very clear that the output of such programs has a great deal to do with who

is choosing the inputs.

If both engineers are competent, the use of a model in my experience merely

makes clear that they are arguing about which design basis or heuristic is most appli-

cable to the situation in question.

FURTHER READINGAnon 1977. Specification for graphical symbols for general engineering. Piping systems and plant. BS

1553-1:1977 BSI Standards.Anon 1988. Engineering diagram drawing practice. Recommendations for general principles. BS 5070-

1:1988 BSI Standards.Anon 1999. Symbolic representation for process measurement control functions. BS 1646 BSI Standards.ISO, 2001. BS EN ISO 10628:2001. Flow diagrams for process plants—General rules. BSI Standards.Kauders, P., 2014. Plant Design. How Much? The Chemical Engineer, London.Kletz, T., 1999. Hazop and Hazan. Institution of Chemical Engineers, London.Sinnot, R.K., Towler, G., 2005. Chemical Engineering Design, Vol. 6. Butterworth-Heinemann,

London.






CHAPTER 4

Twenty-First Century ProcessPlant Design Tools

GENERAL

There is a far greater use of computers in process plant design than when I started in

practice. Hand drafting of most drawings is no longer really practiced, and handwritten

calculations are also uncommon. Neither do we need to write our own computer

programs as I had to if I wanted a computer to do something for me back in 1990.

There is also some use of modeling and simulation programs to support design

activity, especially in certain sectors. This has not, however, replaced design activity,

and it cannot, for reasons I will explain in this chapter.

Professional plant design engineers worldwide and across sectors only use a small

subset of the available programs, largely for reasons of economy and consistency to

allow information sharing.

We more or less all use MS Excel, MS Project, and Autodesk AutoCAD.

Modeling and simulation programs tend to be sector-specific specialist products. Oil

and gas industry specific modeling and simulation programs tend to be popular in

universities, but there are equivalent specialist programs (such as the Hydromantis

products used in my sector) which never seem to feature in university courses.

Researchers in chemical engineering departments make a lot of use of simulation and

modeling programs in their research, and they have consequently started making use of

these programs in teaching the thing they call “design.” Some of these programs (such as,

e.g., Matlab) are never used by professional engineers, and are being shoehorned into a

duty they are unsuited for. Some (notably Hysys) are frequently misused by researchers

unfamiliar with professional practice to fill a gap in their own knowledge.

These programs are highly discounted or even free to academia, and are often

incredibly expensive to commercial users. Thus, many of the computer programs used

to teach the thing known as “process design” in academia may be irrelevant, misused,

or financially nonviable for commercial use.

In this chapter I will discuss a few of the programs actually used by plant designers,

as well as the more popular programs misused by researchers for “design” teaching.

Before I do that, I would like to draw your attention again to how we ought to be

using computers, according to the Institution of Chemical Engineer’s (IChemE’s)

Computer-Aided Process Engineering (CAPE) subject group.


USE OF COMPUTERS BY CHEMICAL ENGINEERS

All the new tools used by chemical engineers are computer based, and the IChemE

guidelines on use of computers by chemical engineers should be followed. The most

important thing to understand about these design tools is that they are intended to

support, rather than replace, professional judgment. The guidelines summarize them-

selves with the following key points:

Management has the overall responsibility for developing appropriate standard proceduresand practices and for ensuring that they are followed.

It is a professional engineer’s legal and professional responsibility to exercise goodengineering judgment in making design decisions and, therefore, to satisfy him/herself regard-ing the adequacy of the information upon which design decisions are based. This means you!

Much of this information is today generated by computer-based systems and so thequality of these systems and the skill and judgment with which they are applied to a designproblem are a critical part of these responsibilities.

The purpose of these Guidelines is to suggest some simple precautions which should betaken to help protect the integrity of proposed engineering solutions and thus to adequatelydischarge professional responsibilities, for example:

What matters is the quality of the engineering decision: focus on “fitness for purpose” ofboth the computer-based system and the data which is fed into it

Assume that everything is “guilty until proven innocent”: you must check and ensure thatthe computer-based model is appropriate to your needs and that the data (including anydata from databanks, etc.) is correctly specified and adequately covers the expected ranges(for example, of temperatures, pressures and compositions).

You must check and ensure that the program has worked successfully and that the resultsare adequate for your purpose: you must satisfy yourself that you fully understand any weak-nesses and that you apply them sensibly and with good engineering judgment

Sensitivity analysis is a key weapon in identifying where the critical problems lie and inassessing their likely impact on your design decisions.

Program development is not a trivial job and to do it well requires special skills and experience.Engineering decisions will be based upon the results generated by these programs. The

program must therefore work correctly and proper records must always be kept.Do not hesitate to seek help and guidance from your more experienced colleagues, from

your support services or even from the suppliers of the systems concerned (and seek it early,not when things have already gone wrong).

Unfortunately, these principles seem not to be commonly understood. Such programs

are consequently being used to carry out tasks they were not intended to be used for.

Worse still, such misuse is sometimes actively taught in universities as proper practice, and

engineering employers consequently have to actively reeducate their graduate intake.

IMPLICATIONS OF MODERN DESIGN TOOLS

The workhorse programs allow for great increases in productivity and the possibility of

more decentralized and flexible engineering services. When I graduated, calculations


were done by hand on paper pads by engineers. Drawings were exchanged between

offices in hard copy by courier. Copying of drawings was done by means of a

machine which produced the blue lines on beige background known as a “blue-

print” (and a terrible smell of ammonia). Fax might be used to transmit A4 copy.

There might be one PC shared between a dozen engineers, and time had to be

booked on it. If you wanted a program, you needed to write it yourself, so it tended

to be a bit buggy.

Now, an engineer working alone at a PC can research alternatives, carry out his

own calculations with reliable and extensively debugged programs, generate his own

drawings on his computer, and collaborate with others worldwide using more or less

instantaneous communications. He can send and receive editable versions of his draw-

ings to and from Australia in seconds.

This universal use of computer-aided design (CAD) and web-based communica-

tions has also had the effect of closing the drawing offices which were a feature of

engineering companies back when I started. There are now drawing offices in India

and elsewhere which will produce your drawings for you at a fraction of UK drafting

rates, but even they rarely feature drawing boards.

The general implication of all modern design tools is that they can harness a great

deal of stupid, patient computing power. This can be used to produce models of pro-

cess plants in Excel, dedicated modeling and simulation programs, or even programs

like Matlab which can be used to throw ourselves back to the time when we had to

write our own programs.

So we can use computers to produce reliable, and transparent models, or models

which are rather opaque to all but the most experienced engineers. Just as banks will

only lend money to people who don’t need it, the only people who should use many

modern design tools are those who don’t need them.

Both lecturers and students often think that because modeling and simulation

programs work on the basis of first principles, their output is somehow more rigorous

or better understood than heuristic design. I am, however, fairly sure that even those

who write such programs do not completely understand them, as no one can

completely understand a computer program beyond a fairly low level of complexity.

I am also fairly sure that those who write these programs would not understand the

plant “design” produced by such programs, even if they did understand the program

itself, for similar reasons of complexity.

Those who think that the output of such programs is more trustworthy or trans-

parent than professional approaches do not understand that all approaches are approxi-

mate and heuristic, but that the professional approach is based on producing a model

simple enough for complete human comprehension, founded as directly as possible in

empirical evidence and professional judgment, and tested at full scale by generations

of professional process designers.

55Twenty-First Century Process Plant Design Tools

The modeling-as-design approach on the other hand is based in a necessarily cut-

down version of pure theory, running on a desktop computer in a model which no

one fully understands, written by people without any experience in designing process

plants, untested at any scale, and never intended to be used as a substitute for real

design calculations.

CATEGORIES OF DESIGN

Unit operation sizing and selectionUniversities still teach students approaches to unit operation sizing based on hand

calculations using charts which come from the slide-rule era. Many who do this think

it helps students to understand the unit operation—whether this is true or not, it isn’t

much to do with modern professional practice, since process plant designers mostly

use spreadsheet programs to do their calculations.

Equipment suppliers are the ones who do detailed calculations to specify unit

operations (or more likely punch numbers into a proprietary spreadsheet or program),

so that what goes on in universities is at best a relic of an earlier era.

Since spreadsheet programs can’t read charts, many of these techniques aren’t of

much practical use any more. We need equations rather than charts to allow us to

work with spreadsheets.

Modeling and simulation programs are also used to “design” unit operations, in as

much as they are used to model a number of unit operations with a view to writing

specifications for the equipment.

This kind of “design” is far more common in operating companies than in the

contracting companies or design houses who design complete plants and offer process

guarantees for them. Even in operating companies, simulation programs are supposed

to support design activity rather than substitute for it.

Operating companies have access to large quantities of real plant-specific data, so

they can tune and validate the simulation programs to make them match their plant.

Whole-plant designers do not have this luxury, as their plants have not yet been built,

and they lose access to operating data for the plants they do build once commissioning

is completed.

Hydraulic designIn the hydraulic calculations used to size pumps and pipework, empirical approxi-

mations like the Colebrook�White have superseded the Moody diagram, for the

reasons discussed in the last section—MS Excel can’t read diagrams.


The modeling and simulation programs used for plant “design” can usually do

some hydraulic calculations, but often come with a set of defaults which causes

problems for the unwary or uncritical user.

There are quite a number of fluid dynamics modeling programs which can be

used for complex fluid dynamics problems, but these are a bit overpowered and slow

for the most common hydraulic calculations.

Mass balanceI would expect that anyone reading this book would know what this is but, just in

case, process plant designers work out how much stuff is going to flow from one place

to another in their plants by applying a simple principle—if matter is neither created

nor destroyed, all the masses of stuff must add up.

This “mass balancing” is usually done by practitioners using MS Excel. It is partic-

ularly common and helpful to have one unit operation per tab, and have the mass

balance expressed in making links between tabs. My suggested methodology is

explained in Chapter 13.

Energy balanceEnergy is not created or destroyed either, so we can work out the energy needs and

yields of our plant by balancing energy inputs and outputs.

The energy balance is usually built into the same Excel spreadsheet as the mass

balance.

TOOLS—HARDWARE

Mobile devicesPrice d1�500

More or less everyone has got one of these with them all the time nowadays, and

they all have a basic calculator built in, which is perhaps why no one can do mental

arithmetic any more.

You might need to have pi and a few other numbers like root 2 and root 3 memorized

to a few decimal places (or you can google them with your mobile in the case of my

students), but I find that the calculator can do most of my back-of-an-envelope calculations.

Many can also run Excel-compatible spreadsheet programs for when you want to

do something a little trickier, and have a permanent record of your calculations which

you can tweak up in Excel later.


When I asked my students to evaluate graphical calculators, one asked me why

they would pay d200 for one when everybody already has a mobile device which can

do pretty much everything it can, as well as many other things as well.

Handheld calculatorsPrice d10�20

These are the slide rules of the modern era, but mine doesn’t make it out of my

desk drawer very often nowadays.

Sums I can’t do in my head are more likely to be done with my mobile phone

than with my far more powerful scientific calculator. Anything remotely tricky gets

done in a spreadsheet, which allows for easy checking, and automatically records the

output of the calculation.

I have a friend who is an enthusiast for the graphical calculators now commonly

available, but usually banned from school and university examinations in a way which

limits their use in academia.

Graphical calculatorsPrice d100�200

My enthusiastic friend tells me that modern graphical calculators can not only

produce full-color graphs, they can also do calculus.

I guess they might be very handy once you have mastered their operation, but

they are ultimately a kind of handheld calculator and there are reasons why my

conventional one doesn’t see much daylight.

The use of a spreadsheet program on a PC automatically produces a checkable,

annotated, and fairly permanent record of the calculations you have carried out, and

the assumptions made in doing so. Quality Assurance (QA) and traceability of docu-

mentation is very important in professional practice.

It turns out that these calculators are easily linked to a PC and import and export

commonly used file formats. I looked into a couple of models which were recom-

mended to me: the TI Nspire and the Casio FXCG20.

The TI calculator solves differential equations, which the Casio unit does not, but

TI have pulled these calculators from the UK market due to lack of interest, so it is

hard to get hold of one.

Both look like they might be pretty good teaching tools, as they come with an

emulator which runs on a PC which you can use to demonstrate their use. I’m not

sure what advantages they would give to a practitioner though.


TOOLS—SOFTWARE

PCs do most of the heavy lifting nowadays in process design. Engineers have to care

about the price of products so, in the course of researching this book, I attempted to

obtain prices for all the programs I looked at.

This was trickier than I thought it would be and, consequently, many of the prices

quoted in the following sections are approximate. There seems to be a culture of

secrecy about pricing in the sector, and the generation of confusion with complex

price lists.

I think, however, that the prices quoted are approximately correct for similar

functionality.

SpreadsheetsMS ExcelSingle user license d110.

This is the workhorse which does most chemical engineering calculations. I’m not

recommending Excel; I’m just noting that it’s what everyone else uses. There are

competing products like Openoffice.org’s “Calc,” which do pretty much the same

things, but are free.

I used to prefer Lotus 123 back when it was a viable alternative but, as is so often

the case, the most commonly used product drives out the competition, if only so that

engineers have a common file format to collaborate with. Lotus 123 only went off

sale in 2013, but it became a hassle to use it from a collaborative point of view around

the year 2000.

The Excel product as it comes out of the box is pretty powerful and versatile, but

it has a more powerful tool still contained within it (Visual Basic)—see the next

section.

Many of the software tools used and taught in academia have no functionality for pro-

fessional engineers greater than that offered by Excel, and are not as transparent as Excel.

It may be a pain to grind through someone else’s spreadsheet checking that all the

calculations are set up right, but at least you can, and you are certain that the program

itself will do what is asked of it.

Naming Excel tabs appropriately facilitates the checking of calculations, and is

practiced by most engineers, but many are still unaware of a feature which helps even

more: naming cells. This makes self-checking (and more important still checking by

others) a great deal easier.


http://www.openoffice.org/

To the left of the formula bar above the worksheet is the name box. You can use

this to give individual cells or cell ranges a name (Figure 4.1).

If you do this the formulae in your spreadsheet will have a format which allows

you to check that they are correct without having to roam all over a multipage

spreadsheet to see if the right box has been referenced.

To give a simple example, labeling cells C8�C10 with a description of their

contents:

Volume5 height3 PIðUÞ3 ðdiameter=2Þ2

Is a lot more transparent than:

C105C83 PIðUÞ3 ðC9=2Þ2

For similar reasons, I avoid simplifying the equations I put into Excel. I create the

terms in the equations within discrete brackets from obvious sources. This looks

pretty ugly and unwieldy, but we should always think of someone else having to

check your calculations, as well as perhaps having to come back to them in 15 years’

time yourself.

Figure 4.1 Screenshot of a blank MS Excel spreadsheet.


Despite these simplifying measures, there will still be implicit assumptions or a

required way of using spreadsheets which others might not know about, and we might

forget over time. We should annotate the spreadsheet using comment boxes with this

information, at its point of use. Separate documentation or instructions for use are

much more likely to be lost over time than embedded versions.

Microsoft Visual BasicPrice: free with MS Excel.

Back when I started with computers, if you wanted them to do anything, you had

to write the program yourself. Most of us began with a language called Basic, a

cut-down version of the venerable Fortran.

Microsoft’s version was called GW Basic (GW was commonly thought to stand for

“Gee-Whizz!”—they are Americans after all). GW Basic’s modern descendant is MS’s

Visual Basic (VB).

It allows us to automate spreadsheet functions, and write programs to do things

which standard Excel cannot. However, this means that we are doing programming,

in other words doing “a nontrivial job. . . requiring special skills and experience.”

Those without these skills and experience need to consider the two main aspects of

checking computer modeling programs: verification in which we check that all the elements

are correctly coded, and validation in which we check that the model matches reality.

Past a certain (quite low) degree of complexity, computer programs do things we

didn’t expect them to. The best fully verified and validated commercial programs are

thought to contain around 4% undiscovered errors.

Our own programs should be assumed to be far faultier. Writing your own

program is rarely going to be the quickest way to solve a practical engineering prob-

lem, when the necessary validation time is considered.

That said, Douglas Erwin has written a useful book on how to use VB to assist

with carrying out real design tasks (albeit only in industries based on organic

chemicals)—see Further Reading at the end of the chapter for details.

If you have time for proper verification, writing programs in VB becomes a viable

approach. This is usually only the case if the program is to be used many times. For

example, I used some VB in writing the Excel spreadsheets I use for hydraulic calcu-

lations (of which there are dozens on every plant design I carry out), so it was worth

paying to have the spreadsheets third-party beta-test verified.

I have used these programs with confidence hundreds of times since they were

verified, even though the initial effort of producing them and having them checked

was far more than I would have expended on manual calculations for a single project.

One-off programs are unlikely to be economic to properly test and verify, as

Erwin acknowledges when he tells the reader that the VB programs which come with

his book are not tested, and are consequently likely to generate error messages which

he challenges users to fix for themselves.


Other numerical analysis softwareMathWorks MATLABSingle user license—depends on options, but certainly d10K1 .

Matlab is commonly used and taught in academia, though hardly anyone uses it in

professional chemical engineering practice. It underpins Simulink, a program which

allows you to write your own dynamic modeling software (just like the IChemE tells

us not to).

The IChemE’s guidance on use of computers tells us why practitioners should not

write their own models—unverified programs are error prone, and unvalidated

programs are misleading.

It will undoubtedly be quicker to use a commercial program than to write, verify,

and validate our own from scratch, but of course modeling is not design in any case.

As programs, Matlab and Simulink both may be fine when used as intended, but

they just do sums. They do not do engineering.

PTC MathcadSingle user license around d1,000.

Mentioned only for completeness, Mathcad mainly just does algebra. I have never

seen it used in professional practice, probably because engineers can do their own

algebra. It integrates with Creo, a drafting program from the same vendor, but process

plant designers don’t use Creo either.

Simulation programsThe dividing line between use and misuse of simulation and modeling programs is

whether the IChemE CAPE guidelines are followed, and particularly whether model

verification and validation is undertaken.

If you have a great deal of applicable data on the exact plant you are designing,

and are designing many similar plants, you can go to the effort required to fit a

modeling program to your plant, and write accurate models of your unit opera-

tions. Plant design then becomes a question of linking these blocks into an

integrated model, and optimization of the model can be a valid proxy for optimiz-

ing the plant.

If, however, you are doing a one-off design, you do not usually have a great

deal of information about the plant which will be built. Rather than using a

validated model, you will be using the straight-out-of-the-box generic data and

models, and optimization of this unvalidated model makes no sense. The errors in

the model are very likely to be greater than the resolution of the optimization

procedure.


Plant operators are the ones holding the information necessary to validate and

tune modeling software. The contracting companies who design the majority of pro-

cess plants do not have this information, and consequently make less use of modeling.

This is presumably why the most commonly used modeling software is written to

support the oil and gas companies who are best placed to put in the data, time, and

effort needed to validate modeling software.

There are quite a number of modeling programs, so I shall restrict myself to the

most popular ones in the discussion to follow.

Aspentech Hysys, etc.Single user license d10�20K.

Hysys is clearly written for process optimization in the oil and gas/petrochemicals

industry, though it is nowadays commonly misused as if it were a design tool for all sectors

in academia. Many green graduates I meet cannot attempt to “design” a plant without it.

If you google “Hysys,” you will find Aspentech’s site (where it is described in

terms which match my understanding of its purpose), and a great many other sites by

academics, where it is described as a design program of use across all process sectors.

Invensys SimSci Pro/IISingle user license d15K.

A steady state process simulator which is, I am told, perhaps more popular with

contracting and operating companies in the oil and gas industry than Hysys.

The manufacturer’s website states that “PRO/II offers a wide variety of thermo-

dynamic models to virtually every industry,” but its available unit operations are mostly

limited to those of the oil and gas industry. As they themselves state:

Spanning oil & gas separation to reactive distillation, PRO/II offers the chemical, petroleum,natural gas, solids processing and polymer industries the most comprehensive process simula-tion solution available today.

Pro/II is, like Hysys, used as an optimization and debottlenecking tool in that

industry, and it is similarly misused in academia to attempt to replace proper process

design.

Chemstations CHEMCADSingle user license d9K.

This was the simulation program I learned to use in university back in 1990.

I never used it in practice, and have never seen it used in professional practice or

academia, although the supplier’s website makes it clear that it is still in production.

It looks to do the same things as Hysys, etc., and I have no reason to believe it

does them any better or worse, other than its seeming lack of use in practice.


COMSOL Multiphysics, etc.Single user license around d15,000.

COMSOL Multiphysics does a different kind of thing to the preceding products.

It does not attempt to model whole plants, but does more detailed modeling of smal-

ler systems. It is described by its manufacturer thus:

You can model and simulate any physics-based system using COMSOLs. COMSOLMultiphysicss includes the COMSOL Desktops graphical user interface (GUI) and a set of pre-defined user interfaces with associated modeling tools, referred to as physics interfaces, formodeling common applications. A suite of add-on products expands this multiphysics simula-tion platform for modeling specific application areas as well as interfacing to third-party soft-ware and their capabilities.

The add-ons which are most likely to interest chemical engineers are:

• Chemical Reaction Engineering Module.

• Heat Transfer Module.

• Computer Fluid Dynamics (CFD) Module.

• Mixer Module.

• Pipe Flow Module.

• Molecular Flow Module.

• Optimization Module.

It is claimed to be able to interface seamlessly with Excel, Matlab, AutoCAD, and

Pro/II via other add-ons.

The problem? There is a great deal more going on in a process plant than physics,

however “multi” that might be. It is not the software vendor’s fault that someone

might think that optimizing a highly simplified model of a subsection of a plant is

optimizing the plant, but this is how it is misused in academia.

OtherThere are also proprietary programs written as one-off specialist products which allow

specific issues to be analyzed. They are designed to be more accurate than generic

products, and I am told that, even when stripped of all components not related to

the limited duty they are designed to, they may take weeks of processing time on a

modern desktop PC to reach a solution.

Project management/programming toolsPlant designers need to be able to analyze and communicate the coordinated tasks

which will be required to design, procure, construct, and commission the plant if they

are to do accurate pricing calculations.

They will usually use the same tools for this as project managers will later use to

keep the project on track and on budget, so the programs are usually a little over-

powered for the use which plant designers will put them to.


Microsoft ProjectSingle user license d550.

The MS program is fairly commonly used by plant designers. Although other, more

powerful specialist programs are probably more common tools for specialist project

programs, everyone’s familiarity with MS products makes “Project” easy to pick up.

Microsoft ExcelSingle user license d110.

You can produce rough project programs in Excel, and sometimes it is expedient to

do so as not everyone has Project, given that it is not included in all versions of the MS

Office suite. Programs produced in Excel are not, however, really up to a professional

standard of presentation, so it is preferable to use MS Project as a minimum standard.

Of course, you can export your MS Excel program into MS Project if you have

formatted it correctly, as well as exporting MS Project data to MS Excel.

AMS RealtimeSingle user license around d1,000.

I only mention this because it is the direct descendant of the program I learned to

write project programs with, Artemis Schedule Publisher. Unlike Schedule Publisher,

I have never seen Realtime used by a plant designer.

Oracle PrimaveraSingle user license around d3K1.

A far more sophisticated program than MS Project, which seems to have super-

seded Schedule Publisher in the sort of companies I work for. As the promotional

literature states:

Primavera P6 Professional Project Management, the recognized standard for high-performanceproject management software, is designed to handle large-scale, highly sophisticated andmultifaceted projects. It can be used to organize projects up to 100,000 activities, and it pro-vides unlimited resources and an unlimited number of target plans.

Not really for use by the plant designer, it is more for project managers, usually

via a specialist project programmer, but if working in a company with such a specialty

program, designers may use the program by proxy.

Computer-Aided Design (CAD): drawing/draftingAutodesk AutoCAD/Inventor, etc.Single seat license for basic package around d5K.

As with MS Excel, it doesn’t matter whether AutoCAD is the best program—it’s

the one everyone uses, and all serious competitors make sure that their programs can

export to Autodesk’s dxf (Drawing Exchange File) format.


AutoCAD used to be a bit hard to learn because, for a long time, they resisted the

now-standard (from MS products) meanings of mouse-clicks, return key and so on,

but now the program allows you to use these as well as supporting the old-school

“draffies” who still use it as if it were running under MS-DOS.

AutoCAD comes in a number of specialist flavors, with preinstalled content and

other customizations suitable for various engineering disciplines, as well as a version

specially for drafting P&IDs to US standards, though a company called Excitech will

give you free files (see www.excitech.co.uk) to make “AutoCAD P&ID” compliant

with British Standards.

Bentley Systems MicrostationSingle seat license for basic package around d3K.

Whether Microstation or AutoCAD is the better product isn’t the question—

AutoCAD dominates the market. In my opinion, Microstation does itself no favors by

being consciously so different from AutoCAD, such that there is a steep learning curve to

master the most basic functions of what will probably always be the second-banana product.

Other than that Microstation is a perfectly good program, whose main advantages

are keeping AutoCAD on its toes, and listening to its users. Virtually no one uses it for

process plant design. Microstation has built-in simulation and modeling capabilities while

AutoCAD does not, for people who like that sort of thing. (You know, theorists.)

PTC CreoSingle seat license for basic package around d4K.

Formerly known as Pro/Engineer, this is not widely used in process engineering.

It is far more popular with those who do 3D drawings such as product and mechani-

cal designers and architects.

Computer-Aided Design (CAD): process designBentley Systems Axsys.Process/PlantWiseSingle seat license for basic package around d12K.

The suppliers say that it has been designed to allow rapid Front End Engineering

Design (FEED) studies, and they describe it as follows:

Interfaces with all the major process simulators, including HYSYS, AspenPlus, UniSim, and Pro/II, so you can use your system of choice and properly manage the resulting data. Data frommultiple simulation runs can be easily compared and new design cases quickly generated

Automatic creation of Process Flow Diagrams (PFDs) and Piping & InstrumentationDiagrams (P&IDs) using project-specific symbols and drawings that can be output to multipledrawing formats

Integration with the major heat exchanger applications such as HTRI and HTFS for fasterheat exchanger design


http://www.excitech.co.uk

Use of Microsoft Excel to provide easy data entry and generation of intelligent datasheetsand reports in your specific formats

A managed workflow that incorporates graphics, data, and work management for eachuser and tracks changes during the project, allowing users to revert to previous designs

Intelligent documents and data that can be transferred into detailed design products,including Bentley AutoPLANT and Bentley PlantSpace

Improved Front-End Engineering Design, which helps reduce capital expenditure as proven byusers.

It also allows automatic pipe routing. This sounds like a great product, so I’m not

sure why I had never heard of it before I researched this book.

Having an integrated suite of programs which combine drafting and design with

modeling and simulation outputs sounds like an excellent idea on paper (especially for

those following the academic modeling-as-design approach), but I wonder if each of

the bits does its job as well as a dedicated program, or a professional engineer.

Computer-Aided Design (CAD): hydraulic designComputational fluid dynamics (CFD) allows the visualization of complex patterns of

fluid flow in a physics-based model. The output is often pretty, but does not necessar-

ily match reality that well.

CFD isn’t much to do with whole-plant design in any case, but I’ll mention them

in passing for those who think it is.

There may be occasions where CFD might come in handy to design a particularly

challenging element of the plant but you’d be wiser to buy the service in from a

specialist, than learn on the job.

COMSOL MultiphysicsThis is one of the functionalities of the COMSOL product mentioned in an earlier

section.

Matlab/SimulinkYou can write your own CFD program using Matlab. You probably shouldn’t though.

Autodesks Simulation CFDThis dedicated program seems to offer some of the same fluid flow and heat transfer

analysis features as COMSOL Multiphysics and Matlab. Being an Autodesk product,

it presumably integrates well with AutoCAD.

OtherMicrosoft VisioSingle user license d300.

Visio is flowchart software which is not intended for producing professional engi-

neering drawings, but, like all MS products, it is easy to pick up, so it has become


commonly used in academia as a substitute for the professional drawing programs

most academics can’t use.

It does have a good-quality “export to dxf” feature which allows its product to be

brought in to AutoCAD for professionalizing, so all is not lost if beginning designers

can only use Visio, but I personally don’t teach it at all, as it is much simpler to go

straight to the far superior AutoCAD.

I have seen it used by professional engineers in some interesting ways, dependent

on its ability to link to Excel, for example producing dynamic hydraulic profiles

which alter the relative position of drawing elements in response to underlying linked


Microsoft AccessSingle user license d110.

Microsoft’s Database software, MS Access can be useful for document control.

FURTHER READINGErwin, D.L., 2014. Industrial Chemical Process Design. McGraw Hill, New York, NY.IChemE CAPE Working Group, 1999. The Use of Computers by Chemical Engineers: Guidelines for

Practicing Engineers, Engineering Management, Software Developers and Teachers of ChemicalEngineering in the Use of Computer Software in the Design of Process Plant. Institution ofChemical Engineers, London.







CHAPTER 5

The Future of Process Plant Design

PROCESS PORN

In my intercourse with mankind, I have always found those who would thrust theory intopractical matters to be, at bottom, men of no judgment and pure quacks.

John Smeaton

Some areas of physics have, to outsiders, clearly lost themselves in abstraction.

Seduced by the beauty of higher mathematics, they pursue things which seem to look

right, even though they are piling unsupported speculation on top of itself many times

over to get there.

The theorists responsible are now crying to be freed from the requirement to

prove any part of their theories empirically. They think that they should instead be

allowed to pursue mathematics and philosophy where they think they lead.

The partial differential equation entered theoretical physics as a handmaid, but has graduallybecome mistress.

Einstein

The problem is that mathematics and philosophy deal with what is plausible within

their conventions, rather than the truth. Without a grounding in empiricism, physics

of this type is pure self-indulgence, which is why the product of this lost school of

physics is sometimes called “physics porn.”

The philosophical tool which protects us against losing ourselves in abstraction in

this way is the beefed up version of Occam’s Razor known colorfully as “Newton’s

Flaming Laser Sword”: “what cannot be settled by experiment is not worth debating.”

We seem to have developed a similar problem in process plant design. In a

computer model’s mathematical space, many things seem plausible, but we only find

out what is possible when we build the plant. Some are even generating things they

call rules of thumb for design by repeated simulation, as if we had proven that such

models are reliable analogues of the real world.

This approach is similar enough to physics porn in its lack of empirical support that we

might call it “process porn.” In the resultant academic discourse on process design, it seems

now to be considered axiomatic that the approach followed by all professional engineers is

hopelessly obsolete.

An approach based on higher mathematics, theoretical sciences, modeling and

simulation programs, and network analysis techniques is now thought in academia

to be the future of process design. This may have something to do with the fact


that these disciplines and tools are those which academics know and, further, that the

vast majority of chemical engineering lecturers worldwide have never designed a real

process plant.

The idea that engineering is just applied mathematics and science is commonplace

in these circles, but this is an idea held only by those who have never practiced, or

have never reflected upon their practice.

All practitioners know that much of what they were taught in university is worth-

less in practice, and much that would have been of value was not taught (the supposed

exception to this rule are French engineers, who according to the old engineering

joke ask “So eet works in practice, but does eet work in theory?”).

Universities feel that they have to staff their departments with scientific researchers,

and few engineers want to do research—we want to be engineers, not scientists.

Scientists are also a lot cheaper than chemical engineers. All of this was fine as long as

staff knew that they were stand-ins for the engineers who were not available or afford-

able, but they have started making a virtue of their deficiencies.

As noted in a previous chapter, I hear that many research-led universities teach the

following as a process plant design methodology:

• Look, in scientific literature, for bench-scale experiments which give possible pro-

cess chemistry.

• Use these unproven techniques as the basis of a costing exercise which goes only

as far as comparing feedstock and product prices.

• If the product sells for more than the feedstock, assume the process is economic.

• Use the unproven technique as the basis of a hysys model.

• “Optimize” the hysys model (which in this context means only getting recycles to

converge).

• Grind through pinch analysis by explicitly defined rote methodology.

• Produce a short Word document which describes how they navigated the decision

tree provided by the lecturer.

At the end of this time-consuming exercise, all we have is a worthless hysys model

based on bench-scale experiments without any scale-up consideration, limited in

scope to a reactor and an associated separation process.

We have given no thought to whether the standard hysys data and assumptions are

valid in our case (they will probably not be), and we apparently think that getting

recycles to converge in a computer model can be called optimization.

We have not required students to give any thought to cost, safety, or robustness,

produced no engineering deliverables, and they have at no point been required to

exercise the slightest judgment, imagination, or intelligence.

This state of affairs is not just useless, as it teaches the opposite of professional practice:

• Engineers don’t ever use lab research as the basis of full-scale plant design.

• Engineers don’t ever use modeling in place of design.

• Engineers don’t ever ignore cost considerations.


• Engineers don’t ever ignore safety considerations.

• Engineers don’t ever produce “designs” that no one (including them) really

understands.

• Engineers design by producing recognizable engineering deliverables.

In direct contrast, this academic approach represents, as I understand it, best

practice according to researchers in “process design.”

It is ultimately based upon approaches originally intended to allow beginners with

no experienced supervisor to attempt a certain rather unrealistic and outdated kind of

process design; approaches which have been stretched far beyond their originally

intended purpose.

Some parts of the approach do have limited use, mostly in optimizing existing

plant in certain industries, but these too are used out of context and without valida-

tion in the real world.

So let’s unpick some of the ideas underlying the academic approach, and then

consider some further questions about the future of plant design, bearing in mind

how much of the core of process plant design is the same today as it has always

been.

WILL FIRST PRINCIPLES DESIGN REPLACE HEURISTIC DESIGNIN FUTURE?

In a word, no. All is and always will be heuristic in the foreseeable future of engineering

design.

We know this for sure because heuristic design is enshrined in law worldwide, and

with good reason. Codes of practice and national and international design standards

require heuristic design calculations to be carried out for safety purposes.

More theoretically, despite the hubris of some scientists, science has simply not

advanced to the level where it can describe in sufficient resolution all the complexities

of a proposed process plant.

Companies also have their own design manuals which require heuristic design

checks to ensure that designs include the company’s know-how.

Some companies now use tailored simulation program blocks representing the unit

operations they most frequently design to carry out this duty, but they build into these

blocks in-house empirical information obtained from past designs. This is not first

principles design; it is empirically validated heuristic design. The program is just a

container and vector for the empirical data, and its output is checked for sensibleness

using further heuristics.

A process plant is too complex to be sufficiently fully described by any first princi-

ples model simpler than the plant itself, and any sufficiently complex model would be

too complex to understand.

71The Future of Process Plant Design

The purpose of heuristic design is to produce a good-enough model of the plant

encompassing the state of the art which the process designer fully understands. A model

which is slightly more accurate than the good-enough one is actually worse than one

which is slightly less accurate, because it will also be slightly less well understood.

WILL PROCESS DESIGN BECOME A FORM OF APPLIEDMATHEMATICS IN FUTURE?

This is the process pornographer’s ideal, but it’s just not going to happen, for the

reasons given in the last chapter.

Process plant design will no more become applied mathematics than medicine

will, as any practitioner understands, because:

As far as the laws of mathematics refer to reality, they are not certain; and as far as they arecertain, they do not refer to reality.

Einstein

WILL PRIMARY RESEARCH BECOME THE BASIS OF ENGINEERINGDESIGN IN FUTURE?

The academic papers freely available within academia are usually too expensive to

access for practitioners outside the paywall, but even if we had free access to the

papers, we understand how many high hurdles there are between bench and plant.

Academics frequently mark student design based on bench-scale academic research

very highly, as it satisfies their desire for radical novelty, but engineers know that few

things scale up well from bench to plant.

We do not give high marks for novelty—we give high marks for a working, safe, and

cost-effective plant. We give low marks (i.e., fire you) for designing a novel but unsafe/

unreliable/loss-making plant. The judiciary may also give you low marks in court.

So: No! Engineers are not going to start spending millions of dollars to build a

plant which scales a process up by a factor of 100 from a bench-scale experiment any

time soon.

WILL “CHEMICAL PROCESS DESIGN” REPLACE PROCESS PLANTDESIGN IN FUTURE?

The future of process plant design is envisaged by the IChemE in “Chemical

Engineering Matters” as being to do with providing for human needs—food, water,

medicine, and energy.

Douglas’s original “Chemical Process Design” is based on a set of assumptions

which do not hold in many of these sectors, and answers different questions to those


asked of process designers in these areas. The school of Chemical Process Design

developed from Douglas’s ideas by other academics is less useful still in these sectors.

Some aspects of this approach are, however, finding favor in the petrochemical

industry which matches most closely the basis of many of its embedded assumptions.

This need not trouble us much in the developed world, as it is uneconomic to build

new plants in this sector at our wage rates. The main use for its component techni-

ques and tools is still the optimization, debottlenecking, and minor modifications to

existing plant, even in that sector.

WILL NETWORK ANALYSIS FORM THE CORE OF DESIGNPRACTICE IN FUTURE?

Network analysis is all very clever, but it can only do one thing. Setting in stone the

results of such an analysis as the foundation of design is very unlikely to be optimal.

1. The more integrated a plant is, the less controllable (and by implication safe) it is,

and the harder it is to start the plant up. This costs the client money in commis-

sioning and maintenance engineers’ time, the provision of back-up equipment for

start-up and so on.

2. In the specific case of heat integration, it should be noted that heat exchangers are

not free. Putting in enough heat exchange capacity to approach the theoretical

maximum possible heat recovery (as many academic network analyses suggest) will

never be economically sensible. The interpretation of sustainability which is used to

support such an approach runs counter to that in the IChemE’s sustainability metrics.

This approach can have value in optimization of an existing plant, but in order to use

it at conceptual design stage, we have to apply it to “optimize” an unvalidated model.

Since optimizing such a model does not necessarily optimize the plant which is built,

this is an example of mismatch between a design technique and the resolution of the

model to which it is applied.

WILL PROCESS SIMULATION REPLACE THE DESIGN PROCESSIN FUTURE?

Rather than being the future, this is a very old idea, the fond hope of academics since

computers were first devised.

Simulations are made of mathematics. Mathematics is perfect, but the real world is

made of rather more complex and imperfect stuff, and contains even less perfect, even

more complex people.

Materials and feedstocks are never perfect. Equipment is never perfect, and

becomes increasingly imperfect over time. Operators are never perfect, and have a


tendency to become increasingly imperfect over time unless well managed. Plants are

never constructed exactly as per the original design.

I cannot find any research papers in which modeling and simulation programs are

used in their straight-out-of-the-box format to design a plant, and the predictions of

the model then validated against the real plant.

The most impressive support I can see for the validation of this approach is that after

such programs are “calibrated” with large volumes of full- or pilot-scale plant data, they

can predict performance of small sections of plant to a reasonable degree of accuracy.

This is, however, simply using the program to contain empirical data, with the

program itself only filling in small gaps in the data, and even this approach has only

“worked” with a couple of linked unit operations at a time, rather than a complete

plant. A professional designer could probably have produced the design of a complete

plant in far less time than it took for even this limited success.

So even if it became possible to accurately model the full complexity of the real

world, it would take longer to program the model than to simply design the plant,

and the part of the process design which a simulation describes is at best the mass and

energy balance, and Process Flow Diagram (PFD).

Only someone who thought that safety, plant layout, hydraulic considerations,

and cost were trivial side-issues would think it plausible that simulation could replace

process design. Process design is in any case one small part of process plant design.

Even if this hurdle was overcome, plants designed by computer would be under-

stood by no one. Plants understood by no one are not capable of verification as suffi-

ciently safe, robust, and cost-effective. Following such an approach would be based on

a misplaced faith rather than reasoned professional judgment.

Even if all of these issues were overcome, these programs do not produce the engi-

neering deliverables which are the immediate point of design. They are not design tools.

WILL PROCESS PLANT DESIGN NEVER CHANGE?

Surely this has to be a “No” too, but some things have been conserved from the very

beginning of chemical engineering.

If the defining concepts of Chemical Engineering, that is, of unit operations, mass,

energy and momentum balancing, quantification and analysis, and so on, are lost, the

discipline will no longer exist, but they have proved useful for some time now.

Similarly, as long as resources come at some cost, there will be selection pressure

to maintain the same stages of design as are common to all engineering disciplines.

As long as the limits on our knowledge of physical sciences and computing power

cause process system complexity to prevent us from making a completely accurate model

of a proposed plant, so the limits of human brainpower will prevent the understanding of


models beyond a certain level of complexity; yet professional responsibility requires us to

understand what we are proposing: modeling cannot replace heuristic design.

But, in my professional lifetime, we have produced many useful new design tools

and, to a far lesser degree, useful new design techniques. Things will undoubtedly

change, but when I look back to what we thought the world would be like in 2014

back in the 1960s, I am hesitant to predict how they will change. I hope it will be as

surprising as that was, though I’d gladly trade the internet for a personal jetpack.

I am, however, happy to predict that when professional engineers do change how

they work, they will change to something which gives a provably safer, cheaper, or

more robust product within the real constraints we have to work under.

FURTHER READINGAlder, M., 2004. Newton’s Flaming Laser Sword, or: why mathematicians and scientists don’t like

philosophy but do it anyway. Philos. Now May/June (46).Anon, 2013. Chemical Engineering Matters. Institution of Chemical Engineers, London.





PART 2

Professional Practice

CHAPTER 6

System Level DesignINTRODUCTION

The very essence of process plant design, the thing people employ chemical engineers

to do, is system level design. By this I mean more or less the same thing as Pugh

means by his term “Total Design,” rather than the approaches called by similar sound-

ing names in academia.

It is integrative, though I do not mean by this the academic “Process Integration,”

or “Process Intensification,” I mean integration of the needs of all engineering design

disciplines, and those who are to build and operate the plant.

It has a broad ranging vision—a good process plant design considers all the elements

of design given in this book, in order to produce an appropriately detailed set of docu-

mentation to allow decision making in early stages, and construction in the last stage.

It is multidisciplinary, involving usually (as a minimum) civil and electrical engineers, as

well as, to a lesser degree, construction stage staff, management, clients, and plant operators.

It is multidimensional, taking into consideration as an absolute minimum the cost,

safety, and robustness implications of every decision.

It is iterative—a design evolves through successively better incarnations.

Lastly, the thing which makes it truly system level design is that process plant

designers see in their mind’s eye a complex system working as a whole. This is why

all partial approaches entirely miss the point—it isn’t about optimizing any one vari-

able. It is about being able to imagine a completely integrated system which no-one

can fully understand, but that the designer understands well enough to make it do

what they want it to do in the way they say it is going to do it.

So how do we do this?

HOW TO PUT UNIT OPERATIONS TOGETHER

The main tools for system level design are the Piping and Instrumentation Diagram

(P&ID), Process Flow Diagram (PFD), and General Arrangement (GA), which repre-

sent a great deal of our design deliberation in a concentrated form. We can see from

them, at a glance, most of the things we need to consider in making the components

of our plant work together.

We will use them in slightly different ways as design progresses, but the PFD

encapsulates the integration of mass and energy balance, the P&ID system level

control and integration, and the GA the physical and hydraulic constraints.


They are not just records of the designer’s thinking, though this is an important

function to allow the review of designs by others. Producing these documents forces

the designer to consider the issues described in the last paragraph, and allows him or

her to visualize the effects of their proposed solutions. They are therefore design tools

as well as records.

None of the academic methodologies intended to serve the purpose of process

integration are used in design practice because they are addressing a problem which

professionals have already solved, in a more comprehensive and universally compre-

hensible way.

MATCHING DESIGN RIGOR WITH STAGE OF DESIGN

At the conceptual design stage, we have at least to get a broad idea of recycle ratios,

as these can have a huge effect on main plant item sizing for certain types of unit

operations, such as reactors and their often closely associated separation processes. This

will involve generating a PFD and associated mass balance.

We need to get an idea of how physically large the plant is going to be, so that we

can see if it is going to fit on the available site. We will need to produce a GA to do

this, and carry out rough hydraulic calculations.

These hydraulic calculations will tell us whether we are going to have a completely

pumped system, or make some use of gravity, a choice which will affect the plant

layout and be seen on the GA. We will need to decide if we are going to use a batch

or continuous process, a decision which will again affect layout and plant footprint.

The P&ID will be affected by these choices, and there are also choices to be made

between software and hardware solutions to design problems, the solutions to which

will appear even on early stage P&IDs.

At this stage we are probably designing unit operations using rules of thumb,

without checking whether the units we are specifying are commercially available. We

probably have fairly sketchy design data, have made quite a few simplifying assump-

tions, and have been given only a few resources to get to the desired endpoint.

Our aim is to see if we can fit the plant on our site, whether it is affordable and

whether it is plausible from the point of view of cost, safety, and robustness. This is an

initial rough screening, which the overwhelming majority of proposals fail. We cannot

optimize such a design due to lack of definition, and we should not try, because we

want to err on the side of caution.

If we are asked to produce a detailed design which we are willing to stand by as a

fairly robust investigation, evaluation, and solution of the vast majority of design

problems and tasks, then we need to look much harder. The same three drawings are,

however, still going to be at the center of the exercise, but now they are primarily

tools for collaboration with others.


System level design optimizes the whole-plant design, considering the implications

of design decisions for our civil and electrical partners, installation and commissioning

engineers, and plant operation and maintenance staff. We can send drawings (especially

the GA which almost everyone can understand) back and forth with our design colla-

borators, installation contractors, and so on to take on board their opinions.

We can use the drawings to conduct design reviews with all interested parties.

This is not to imply committees produce good designs, they do not. A plant

“improved” by including layers of afterthoughts, or features which allow people to

feel they contributed, is very likely to be a suboptimal design.

The process plant designer needs a strong vision and a willingness to challenge any

suggested design modifications, inviting anyone making such suggestions to prove that

they are improvements from the point of view of whole-plant cost, safety, and robust-

ness. A plant “improved” in a way which, for example, maximizes profit or minimizes

risk for the civil or electrical partner alone is unlikely to be optimal.

IMPLICATIONS FOR COST

There are many ways to consider the capital and operating cost implications of

designs. Least capital cost is probably the most popular method, despite its shortcom-

ings, but there are also evaluations based on whole-life cost, total cost of ownership,

net present value, and so on.

A well-integrated design considers cost in the way the designer is asked to consider

cost by the client. We need to set aside our preferences and prejudices and give them

what they want if it is possible to do so.

We can design a least cost plant which operates for the defect liability period plus

one day, or a plant with an excellent whole-life cost. We can design a plant based

on the technologies the client’s staff are used to even though there have been better

technologies for decades.

None of these are wrong approaches, if they are the preference of the people who

are paying. We may attempt to persuade clients to use another evaluation basis, but if

it is not in some way cheaper, we are unlikely to succeed.

Generally speaking, from a capex point of view:

• More robust plant and materials may cost more.

• More automated plants cost more.

• Add-on safety equipment (but not inherently safe plant) costs more.

From an opex point of view:

• More robust plant and materials generally cost less to operate.

• More automated plants cost more to maintain, but save on operator time.

• Add-on safety equipment (but not inherently safe plant) costs more.

Note that from the point of view of both capex and opex, simplicity saves money.

81System Level Design

IMPLICATIONS FOR SAFETY

We tend to consider safety issues based on legal responsibilities and professional codes

of practice rather than client preferences in the first instance.

As discussed in the last section, afterthought safety (or “tinsel safety,” as I used to call it

back when my students used to “complete” a plant design with no consideration of safety

and then decorate it with pretty safety features) costs money (it also reduces robustness),

but system level thinking eliminates risk using techniques such as inherent safety.

The system level approach might actually save more money by eliminating layers

of add-on safety features than the cost of the required safety measures controlling the

risks which remain after minimization, substitution, and moderation of risks inherent

in the design, and limiting the effects of adverse events.

Making the design error tolerant and simple may also be included under the

heading of inherent safety, but these to me are basic design principles with effects

beyond the realm of safety.

Any optimization which is carried out at conceptual design stage needs to address

these paramount issues.

IMPLICATIONS FOR ROBUSTNESS

I was originally going to say here that robustness costs money, which is I think what

most practicing engineers would say without a break for reflection, and is often true

at the level of components, but is not necessarily so at the level of systems.

For example, which is better, a Daihatsu or a Bentley? Well, the Bentley looks

pretty slick, but the Daihatsu has a reliability index of 40, and the Bentley 582

(Average reliability is 100, and the lower the number, the better—see “Further

Reading” for source).

So robustness may be thought by many people to always cost money, but the

robustness of simplicity, and well thought out integration of complex systems always

saves money, and can trump the usually higher costs of robust components.

Clients may specify directly a period of operation of a plant, and/or they may give

tender evaluation criteria which make clear whether they want a low capex, low

opex, or low whole-life cost plant. Professional standards would prevent us offering a

plant below certain standards, though these would be pretty low.

We could, if asked, design a plant intended as a display of wealth, showing that

you just don’t care how much it costs to run, because you can afford not to care. This

is, however, much less popular in process plant design than in car design.

We can offer the same plant availability by using multiple cheap low-robustness

units, or a smaller number of more robust items. The latter is often better from the

point of view of whole-life cost, if not from a capex point of view.


RULE OF THUMB DESIGN

Rules of thumb are one of our main easy to learn tools in managing complexity. Real

rules of thumb incorporate knowledge of the limits of operability, safety, and eco-

nomics. They tell us where a working solution is likely to lie. They also tell us which

approaches are likely to fail.

By “real” rules of thumb I do not mean those things with the same name now

being generated using repeated computer modeling exercises. Real rules of thumb

crystallize experience with real full-scale plant which is very similar indeed to the

plant being designed. Computer modeling is theoretical first principles design unless

the model has been validated against real full-scale plant. You cannot generate experi-

ence from theory, only from practice.

It should be noted that all rules of thumb are specific to a set of circumstances, and

contain implied assumptions. They should not be applied outside these specific circum-

stances without the greatest of caution. Ideally, they should not be applied outside their

specific case at all but, practically, we may have to bend the rules a bit on occasion. If we

do this, we need to know that we are doing it, and reflect this in our degree of confi-

dence in our answer. Complacency has led to disaster on many occasions in engineering.

FIRST PRINCIPLES DESIGN

This is not how professional process plant design is done, because anything designed

from first principles is essentially a prototype, and its operators are test pilots.

If we are forced to carry out first principles design, there will undoubtedly be

teething problems. Such a plant may not be safe, is unlikely to be robust, and will

probably not be cost-effective.

So this is not a professional design approach, and there is a fair chance that its

product cannot be made to work at all. This will come as a great disappointment

to the client who spent large sums of their money on what they thought was a

competently executed design.

This is not to say that there will not be occasions on which you are asked by a

client to do something novel without being given the resources to characterize the

problem well enough to avoid first principles design.

This happens to me fairly frequently, especially now that I have an academic title. It

can lead to some difficult conversations. The following should be markers for possibly

crossing a line in this area:

• Being asked to scale up a process by a factor of more than five;

• Being asked to design a unit operation (or worse still a whole plant) based on

bench-scale tests of kit for which there are no full-scale references;

• Building the first plant of any type or at significantly increased scale.


I am not saying that you should never design a prototype (I have designed a few

myself), but that you need to know that is what you are doing, and not confuse this

R1D activity with normal professional process plant design.

DESIGN BY SIMULATION PROGRAM

Many simulation programs now come with rough costing data built in, but unless you

add your own safety and robustness elements, they are not considered, and you can

easily “design” plants you don’t understand very well with no safety factor.

There is no need for a simulation program operator to keep the plant simple

enough to understand it, or to have a model of any part of the system in their head.

There is no need to consider the requirements of other disciplines or stakeholders,

nor any tools to produce collaborative documents.

These factors should give us grounds for grave doubts about the use of such programs

in anything other than the most tightly constrained circumstances. I simply cannot disre-

commend strongly enough their use for “process optimization” at the conceptual design

stage, ignoring most of the important factors in true system level plant optimization.

Simulation and modeling programs can, however, be of use for equipment suppliers

to specify standard products, assemblies of standard products, or standard package plants.

In this application, the default values in the program are replaced by users with real

operating, thermodynamic, and costing data, so that the program is really operating as a

convenient dynamic repository of empirical data.

There can still be some limited use of imagination and even a little innovation

under these circumstances, but we are in my opinion shading into a modeling

program operator/salesman role here.

SOURCES OF DESIGN DATA

Client documentationUsually our client will give us some documented idea of what they want, and may

well have gathered some data to assist us in our design. Clients, however, frequently

attempt to disavow any responsibility for the information they supply, and try to pass

on all design responsibility to design companies.

Designers will need to do what they can to exercise due diligence in checking that

the client data is correct. There are potential opportunities here as well as problems.

Sometimes the correct information or approaches can be more competitive than the

suggested ones, or an alternative endpoint can be better, safer, or cheaper than the one

the client asks for. Commercially minded engineers (which should be all of us) should

be on the lookout for these ways to get ahead of less flexibly minded competitors.


Design manualsIt is common that companies will specialize in designing certain sorts of plants, and will

have codified their know-how in in-house design manuals. These are often mandated as

the source of information and approaches to be used by the company’s designers.

Going off-piste is at the designer’s own risk, and is frequently a sackable offence. It

also exposes designers to the possibility of legal action in future.

That said, I have worked at places which had bad design manuals based on modeling

program output, and felt a professional obligation to challenge the manual. This does not

always go down well, but we are not automata, and “I was just following orders” hasn’t

historically been a good legal defense. Professional judgment is required of professionals.

StandardsGovernments, national and international standard institutes, and trade bodies also codify

know-how about what works and what does not (especially with respect to safety issues)

in standards and codes of practice. Failure to adhere to codes and standards may be more

serious than failure to follow design manuals. It may be illegal and even imprisonable.

National, regional, and international standards may well have conflicting require-

ments. The choice of which ones to follow is often dependent on the sector which

the plant falls into, or conventions with respect to the type of equipment being speci-

fied. For example, the oil industry works largely to the American Society of

Mechanical Engineers (ASME) and American Petroleum Institute (API) standards,

and air pollution control equipment suppliers may look to German TA Luft standards.

Make sure you are following the most up-to-date version of codes and standards,

and don’t follow codes and standards blindly. Exercise professional judgment. Partial

compliance with a written standard for a good reason is better than slavish compliance

where it is inappropriate.

Don’t play mix and match to get round a tough standard, and have a consistent

philosophy from the conceptual design stage on issues like the acceptable size of

fugitive emissions. Make it a good one, as it is expensive to change it later.

Consistency of philosophy makes for ease of comprehension by operators and other

engineers later in the design process.

Note that in countries with well-developed regulatory environment, regulatory

authorities may specify codes and standards to be followed, and offer their own statu-

tory guidance. In less-regulated countries, the minimum standards consistent with

professional ethics are applicable.

Manufacturer’s catalogues and representativesThis can be a rich source of knowledge. In order to produce a design integrated

at depth, you need to have a very deep knowledge of the parts of the thing


you are designing (or know someone who does). Equipment salesmen are not merely

an annoyance as some designers think. You can work with them to produce very

well-integrated designs.

More experienced engineersFor philosophers the argument from authority is a logical fallacy, but I’m not going

to worry about the opinion of people who doubt whether the sun is going to rise

tomorrow.

For new engineers, discussing their ideas with someone who has the feel for

design through long experience is far better input than the most rigorous mathe-

matical modeling exercise.

We old guys might seem a bit negative to newcomers, as so much of experience is

knowing what doesn’t work, and the practical constraints on plant design, commis-

sioning, and operation.

As neophytes (“noobs”) become more experienced themselves, they come to see

that the constraints are not stifling but are the rules without which the game would

be no fun.

Pilot plant trials/operational dataSometimes we have the luxury of information on the operation of an existing very

similar plant at the proposed site, or at least a similar type of plant at a similar site.

Sometimes we may have data from a similarly sized pilot plant of the proposed

type at the proposed site. Sometimes the data we have is for less similar plants.

We need to be cautious with such data, and to be honest with ourselves about

how much trust to put in it. Scale effects need to be considered. A pilot plant less

than say 20% of the rated capacity of our plant might not be that strong a guide as to

the constraints on operation of a full-scale plant.

Other sites and other technologies may be less similar than we think to our

proposed site and technology.

Such data may be used to validate a computer model. Where this is done well

(as it is done, e.g., by Air Products) this is a great aid to the designer. A validated

model is, however, only true for the plant whose data has been validated. It is no

more capable of predicting the operation of a plant 10 times as large as a physical pilot

plant would be—quite possibly less so.

Previous designsThe same companies which have enough experience to have good design manuals

also probably have staff who have designed previous similar plants, and commissioning


engineers who have commissioned such plants. These are the people who know what

does and doesn’t work.

There will in such companies be access to drawings and calculations used to design

previous plants, costing data, and an opportunity to improve on the last design based

on commissioning and operational experience.

That is why clients like to buy from companies with a track record, and new

designers would be wise to try to work for such companies.

“T’Internet”The internet looks quite different outside academia, as professional engineers rarely

have free access to scientific papers. This is not, however, a big problem: most

scientific papers are about experiments at too small a scale to be of much use to full-

scale plant designers anyway.

A far more valuable use of the internet is the free 24/7 access to manufacturer’s

data, literature, and drawings. Nowadays we can choose unit operations based on

detailed information, and insert an accurate AutoCAD version of the chosen kit into

our drawings without even talking to manufacturers.

A word of caution—the internet is undiscriminating in its content. A feel for the

reliability of internet sources has to be developed. If a page looks like it was produced

by a barely literate teenager it is usually obvious that it is not a reliable source of

information. Obvious puff pieces for equipment manufacturers abound in Wikipedia

articles as well as on their own websites. The most useful questions to interrogate web

pages with are “Says who?” and “Based on what?” If you cannot see solid backing for

claims, best not rely upon them.

One more thing: I would advise you not to go on web forums asking more expe-

rienced engineers the most basic details of how to design the thing you are designing.

They will wonder which Muppet asked you to design that thing when you have no

clue where to start. This reflects badly on both you and your employer.

LibrariesYes, actual physical libraries, with books made of paper—they still have them. Large

public libraries may have handbooks and textbooks which are of use to the designer.

It is also usually possible for alumni to get free library access at the university they

attended. Sometimes it is cheaper to take a trip than to buy a copy of a technical

resource, as these can be very expensive.

FURTHER READING

http://www.reliabilityindex.com/manufacturer.


http://www.reliabilityindex.com/manufacturer

CHAPTER 7

Professional Design Methodology

Aeroplanes are not designed by science, but by art in spite of some pretense and humbug tothe contrary. I do not mean to suggest that engineering can do without science, on the con-trary, it stands on scientific foundations, but there is a big gap between scientific research andthe engineering product which has to be bridged by the art of the engineer.

British Engineer to the Royal Aeronautical Society

INTRODUCTION

Each of the stages of design produces increasingly precise deliverables. We start with

broad sketches and rough back-of-the-envelope calculations and we get down to

drawings accurate to the millimeter and calculations good enough to purchase expen-

sive equipment which will reliably do a specific duty.

So each stage of the design process has a natural resolution, or granularity. It will

be resource-intensive to produce a more precise definition of the design at any given

stage than is conventional, and the additional precision will be of no benefit to those

commissioning the exercise. It will be wasted effort.

One of the mistakes made by early-career designers is to attempt to design unit

operations in great detail at the conceptual design stage. They often find that the

information they need to do the calculation as they were taught at university (at best,

as set out in Sinnot & Towler) is simply not available.

Both information and design challenges are generated during each stage of design.

Much of the information needed to carry out detailed design of unit operations may

never be publicly available, but specialist suppliers will usually have it, or an alternative

design methodology which does not need it.

Newcomers to this approach, coming from the certainty of mathematics and pure

sciences which is taught in many places as chemical engineering, may find it a little

odd, even a bit flaky. Why not institute a scientific research program to resolve all the

uncertainties and collect all the missing data? Why not produce a simulation based on

this program so detailed that the design process is more or less just a process of inter-

rogating the simulation?

Why not? Because this would cost a great deal more, take far longer, and produce

less good results than just having professional engineers design and build the plant,

and no one is going to pay you to do that. For example, a company I used to work

for tells me that they are producing a reliable, empirically validated model of a single


unit operation. After 2�3 years of development, it is almost (but not quite) as good as

the 30-year-old rule of thumb based design approach which they are still using to

actually design plants.

So if you are going to use modeling properly, your competitors can give the client

the information he or she needs for a tiny fraction of the resources you propose to

use, probably even as a “free” favor. Process plant design is a commercial activity. Any

methodology which does not recognize that will remain at best an academic curiosity.

DESIGN METHODOLOGIES

There is really only one design methodology in all design activity, the iterative staged

approach described in this book.

However, designers may use discipline-specific design techniques, design philoso-

phies, and design support tools. In circles where no one designs plants for a living, all

kinds of “design methodologies” can grow up.

Setting aside the academic “Chemical Process Design” methodology discussed pre-

viously, there is a handy list of examples of “Design Methodologies” in Koolen’s book

“Design of Simple and Robust Process Plants” as follows:

• Inherently Safe Design.

• Environmentally Sound Design.

• Minimization of Equipment.

• Design for Single Reliable Components.

• Optimize Design.

• Clever Process Integration.

• Minimize Human Intervention.

• Operational Optimization.

• Just in Time Production.

• Design for Total Quality Control.

He then proposes to roll all of these into one in order to obtain “an optimally

designed safe and reliable plant operated hands off at the most economical

conditions.”

Koolen’s background is plant operation, rather than design. In common with

many academic approaches, it is clear from his description of the process that this is a

technique for design of modifications to hydrocarbon processing plants with extensive

operational data supporting a modeling exercise.

Many of the things he calls “design methodologies” are used in professional prac-

tice, but none of these are really design philosophies or methodologies, nor are they

all universally applicable or without conflicting interactions.

To take one example, “Minimize Equipment” has the subcategory “Avoidance of

more reactor trains by development of large reactor systems.” Theorists, researchers,


and idealists will always favor scale-up (making one big novel reactor) over scale-out

(having lots of small proven reactors).

Professional designers will also consider the time and expense of the implied devel-

opment program.

Scale-up needs to balance the turndown of one large reactor against two or three

smaller reactors. You can achieve a larger turndown with two or three smaller reactors

by turning some off. If the market for your product reduces, and the single larger

reactor has insufficient turndown, the site which went for scale-up rather than scale-

out may close.

Notice also the clash between this approach and Inherently Safe Design’s desire for

small reaction masses. There is simply no way to avoid the need to apply professional

judgment.

I mention this book because I have far more sympathy for his approach than those

proposed by academics without operational experience. None of his “design method-

ologies” are terrible ideas in all settings—they all have their place, though for some of

them it is in process optimization rather than design.

It seems a clever approach to the problem it sets out to address, producing a

hydrocarbon processing plant as simple to operate as a washing machine. I don’t know

if this problem has ever actually come up, let alone if anyone has actually applied the

methodology to it, but it looks (to someone who has never been asked the question)

as a decent place to start answering it.

There are approaches used in operational companies to carry out “designs” of suf-

ficient quality to allow operational companies to supervise detailed designs done by

contractors on their behalf. These are the foundation of the approach used in

Koolen’s book, as well as any industry input to academic design approaches. This is

not, however, process plant design as I define it. This is optimization of an existing

design by operating company staff.

I know that there are many theoretical approaches in academia and in operational

companies for the thing they call plant design, but in professional process plant design

practice there is one basic methodology, used worldwide and across sectors. In the

remainder of the book, I will describe this approach in enough detail to hopefully

allow a beginner to master the basics.

THE “IS” AND “OUGHT” OF PROCESS DESIGN

Process design deals with what ought to be. It is not a scientific description of some-

thing which already exists, but a practical creative activity aimed at bringing into

being something thought desirable.

Much of academic discussion of process design is to do with how design ought to

be done, rather than how it is done. Such discussion is normative, rather than

91Professional Design Methodology

descriptive. Without claiming that what is, is what ought to be, I will offer an

approach based on a description of what I believe to be the modern consensus

approach.

What process design ought to be is a way to imagine, select, evaluate, and define

safe, cost-effective, and robust solutions to the problems inherent in the design prob-

lem we have been asked to address. All ways of achieving this are good. We are paid

to be engineers, not ethicists, nor social reformers.

RIGHT VERSUS WRONG DESIGN

There is no entirely right design, but there is an infinite number of wrong ones.

Being sufficiently right really matters if someone is going to spend millions of dollars

on the thing you have designed.

Engineers tend to only design things for construction which they really under-

stand. You might think you can grind through BS PD5500 or EN 13445 and design a

pressure vessel as well as the next person, but if you tell a supplier to make such a ves-

sel to your specification, you are taking responsibility for the integrity of an item

made by others. This is why we tend to make those who supply equipment responsi-

ble for its design.

We need to be able to do enough design to give suppliers’ proposals a quick check

over for reasonableness, but they are far more likely than us to have the know-how to

make a specialist item in a safe and cost-effective way.

INTERESTING VERSUS BORING DESIGN

A good scientist is a person with original ideas. A good engineer is a person who makes adesign that works with as few original ideas as possible.

Freeman Dyson

You need a damn good reason to be interesting as an engineering designer.

“Interesting” or revolutionary design is nowhere near as likely to work as boring old

“normal” design, where there are only incremental changes in design approaches

which are known to work.

Academia is very keen on interesting design, as the research which they are famil-

iar with is judged to a large extent on its interestingness. Practitioners need something

which will definitely work, rather than an interesting research project.

Vincenti gives a number of examples of this phenomenon in his book “What

Engineers Know and How They Know it,” most notably the case of the development

of the jet engine in Germany and the United Kingdom.

After early success with centrifugal engines, the Germans designed an axial flow

engine years ahead of its time, very similar to modern jet engines. There were few


suitable materials to make such an engine at the time, even if they had been working

under peacetime conditions. The German Junkers engine consequently had a life in

service of around 12 h, before it needed replacing with a new engine. It was a terrible

waste of resources. Cost, safety, and robustness had been ignored in favor of novelty

and elegance (Figure 7.1).

The British jet engine designed at the same time was ugly, but it was designed to

be produced under the prevailing circumstances. It was a “boring” design, but it had

a very long service life, and went on to set world speed records (Figure 7.2).

Figure 7.1 The Junkers Jumo 004 engine. Image reproduced courtesy of National Museum of the USAir Force.

Figure 7.2 The Whittle W2-700 engine.


As with all in engineering, there is a balance to be struck. Dyson says “as few orig-

inal ideas as possible.” He doesn’t say “no original ideas.”

CONTINUOUS VERSUS BATCH DESIGN

There is often no real choice if you want to be both right and boring—traditionally

much process design was batch, but equipment tends to be better utilized in a contin-

uous design.

Batch processing is still, however, very common in low-volume/high-value appli-

cations. I have made this section unusually long (with kind assistance from Keith

Plumb from the IChemE’s Pharma subject group), as it is an often neglected area.

Although continuous processing is used to make the high tonnage materials

produced by the oil and gas, bulk chemical, and water sectors, a far greater number

of materials are produced in batch processes than in continuous ones. In a recent

survey carried out in a food ingredient factory, the company was using over

five thousand different raw materials. All the small chemical plants using batch

processes outnumber the total number of plants running continuous process many

times over.

Specialty chemicals, pharmaceuticals, cosmetics, and food are nearly always made

batchwise. Around 25% of chemical engineers work in these sectors, so batch proces-

sing is important to chemical engineers. For some process sectors it is critically

important.

It is not easy to say why batch processing seems to be the poor relation of continu-

ous processes with respect to publications but this is undoubtedly the case. A quick

scan of the index of Sinnot and Towler or Perry’s Handbook shows how little space is

given to this topic.

Sinnot and Towler have three entries in the index, two paragraphs on batch distil-

lation, and nothing on batch heat transfer. Perry does a little better with 14 entries in

the index and several pages on some topics. However, this amounts to considerably

less than 1% of the content of the handbook (as does this section of my book!).

Why use batch processing?Apparent simplicityOn the face of it, batch processes appear to be simply a scaled up version of the pro-

cess that a chemist would use on a laboratory bench.

This makes batch processes attractive if you do not want to spend too much time

and money on development work. It is easy to get from bench to commercial scale

cheaply and quickly if all you are doing is making the kit bigger.


For some sectors, such as specialty chemicals and food processing, the huge num-

ber of products and, in some cases, the short product life cycle means that getting to

commercial scale cheaply and quickly is very important.

For pharmaceuticals, getting a product on the market quickly is important because

of the limited patent life. In general at least half of the patent life is lost during clinical

trials and process scale-up. Batch processes that can be scaled up quickly are at a dis-

tinct advantage.

However, once you look at a batch process in detail, it soon becomes clear that in

practice it is not as simple as it first appeared. The chemistry is frequently poorly under-

stood and the nonsteady state regime of batch processes makes them difficult to model.

Batch processes are therefore quick to scale up but difficult to optimize and their

efficiency is consequently usually far below that of continuous processes.

FlexibilityFlexibility is a great advantage of batch processes. If you have a generic set of batch

processing equipment then it is often possible to make a wide range of products.

Some batch plants make hundreds of different products using similar processing meth-

ods and plant.

Multipurpose plants can be designed for a generic group of products and are fre-

quently designed without any knowledge of the actual products to be made.

This is achieved by using a facility equipped to work with temperatures in the

range 2100�C to 250�C, pressures from high vacuum to 6 bar g and pH from 1 to 14.

Such plants are often made from highly corrosion resistant materials such as glass,

graphite and tantalum, to allow them to handle a wide range of chemicals, unknown

at the design stage.

Solids handlingMany products in specialty chemicals, pharmaceuticals, food, and cosmetics are solids

or semisolids. These products can be difficult and expensive to handle safely and eco-

nomically in continuous processes, particularly at the small scale.

In the case of pharmaceuticals, even the smallest scale commercially available con-

tinuous solids handling equipment may be of the order of 10 times larger than

required. Small-scale batch solids handling equipment is generally much cheaper, less

complex, and easier to maintain than continuous equipment.

Batch integrityOne advantage of batch processing is that it is possible to identify when the processing

of a given sample started and stopped. This means that if the material manufactured

does not meet the specifications, it is possible to reject just the particular batch that

failed without needing to reject other material.


This used to be a highly important part of quality assurance when we depended

on end-of-batch analysis and testing for quality assurance. As more online analysis

becomes available, batch integrity is becoming a less important part of quality assur-

ance and many products are released based purely on the online analysis.

However, there remains a business risk in relying solely on online analysis and

many companies still like to retain batch integrity to minimize their exposure to the

consequences of release to market of out-of-specification material.

The main batch design requirementsThere are two major differences between continuous and batch processes; the non-

steady state nature of unit operations and the importance of time-related sequences of

operations.

One of the major differences is that it is not possible to summarize the details of a

process using a Piping and Instrumentation Diagram (P&ID), as you would with a

continuous process. It is necessary to have other documents to indicate how the pro-

cess changes with time.

Nonsteady stateThe nonsteady state nature of batch processing impacts on all unit operations. To

illustrate the point, the three most important aspects are examined in the next section.

Batch heat transferIf you heat or cool a batch of liquid in a vessel, the temperature difference between

the heat transfer fluid and the batch of material changes with time and so does the

outlet temperature of the heat transfer fluid.

If you have the simplified case of the cooling a homogeneous batch of material

with an internal cooling coil then the heat transfer equation becomes:

lnT1 2 t1

T2 2 t1

� �5

WC

Mc

K 2 1

K

� �θ

where:

T15 initial batch temperature

T25 final batch temperature

t15 cooling fluid inlet temperature

W5mass flow of cooling media

C5 specific heat of cooling media

M5mass of the batch in the vessel

c5 specific heat of the batch

K5 eUA/WC


A5 heat transfer area

θ5 time.

Even for this relatively simply case, the equation is quite complex and, as can be

seen, includes time. For the fairly common case of using an external heat exchanger

and a liquid being fed into the vessel, the equation becomes very complex.

Batch distillationIn the case of batch distillation, the concentration of the liquid in the reboiler (the

batch) will be changing with respect to time as the more volatile components are

driven off. This means that the temperature in the reboiler will rise over time and the

concentration of components in the fractionating column will change with time.

To maintain the required concentration at the top of the column, the reflux ratio

has to be increased over time. A point will be reached where it is no longer possible

to maintain the top concentration, and distillation will have to stop or the top product

be diverted to a separate receiver.

Batch distillation can be used to produce multiple fractions, but instead of the

flows being taken off at different points in the fractionating column, the fractions are

determined by time.

Batch reactionIn a steady state continuous stirred tank reactor (CSTR) the conditions remain con-

stant within the reactor, but in a batch reactor, concentration, temperature, pressure,

viscosity, density, etc. can change with time. An agitator which was appropriate at the

start of the process may be much less suitable at its end.

The contents of the vessel may be more heterogeneous than in a continuous

process, and different parts of the batch will consequently see different reaction condi-

tions during the period of the reaction. This is one of the reasons why apparently

simple batch reactions are in fact very complex. This lack of homogeneity usually

becomes more of a problem as the scale increases and it is increasingly likely that

unexpected reactions occur that have a serious impact on product quality.

Batch sequencingTo be able to calculate the capacity of a batch plant it is necessary to consider the

sequence of operations and whether these operations take place in series or in parallel.

Most batch processing plants have a number of parallel streams of the same series of

operations.

The plant capacity is usually based on marketing demand forecasts for the products

that the plant is being designed to manufacture. Keith Plumb tells me that the only

thing that you can know with absolute certainty about such forecasts is that they will

be wrong.


The design tools for batch sequencing and capacity calculations are similar to those

used for engineering project management, a combination of Gantt and PERT charts.

However, instead of basing the charts on flows of resources, they are based on mass

flows and document the mass and energy balance.

Energy balance and utility requirementsThe energy balance will be based on the heating and cooling requirements for reac-

tions, distillation, and other unit operations, as is the case for continuous plants.

However, the process sequence will determine where and when energy needs to be

input to or removed from the system.

The input and removal of energy will be time-dependent and nonsteady-state,

which makes energy recovery difficult. Heat integration techniques are even less

appropriate to batch process plant design than continuous process design.

Even working out accurate predictions of working utility requirements is impossi-

ble, as the system is too poorly characterized for these to have any certainty.

The simplest (and least wrong) approach is to calculate the maximum utility

requirements for the worst-case scenario of process steps coinciding and then apply a

“diversity factor,” basically a guess of how much of the maximum possible load will

occur in practice based on practical experience of batch processes.

This is not always done well, and some plants are consequently constrained by lack

of utility supply, requiring additional capacity to be retrofitted. In other cases, too

large a capacity with insufficient turndown is supplied, leading to controllability and

efficiency problems requiring remedy.

Despite there being many remaining problems in the field of batch design, it is still

very popular in certain sectors, and there are a multitude of successful batch processes

in operation today.

SIMPLE/ROBUST VERSUS COMPLICATED/FRAGILE DESIGN

There are so many quotes about the importance of this that I am spoilt for choice, but

how about starting with someone considered by many to have been the first engineer?

Simplicity is the ultimate sophistication.Leonardo Da Vinci

Van Koolen says in “Design of Simple and Robust Process Plants”: “A process

plant should meet the simplicity and robustness of a household refrigerator.” This might be a

bit over the top in most cases, but good designs are (in a phrase widely attributed to

Einstein) as simple as possible but no simpler. This is not, as some think, quite the

same thing as Occam’s Razor—it contains an additional note of caution against

oversimplification.


Complex designs shouldn’t be ruled out, sometimes they are needed, but they

should be evaluated bearing in mind the fact that simple plants are easier to under-

stand, easier to analyze (as Popper points out), more robust, more likely to be getting

to the root of design challenges rather than piling afterthoughts on top of each other.

More operable, more maintainable, more commissionable, more reliable, more avail-

able, more robust. What’s not to like?

A classicist turned computer scientist puts it well:

There are two ways of constructing a . . . design: One way is to make it so simple that thereare obviously no deficiencies, and the other way is to make it so complicated that there areno obvious deficiencies. The first method is far more difficult.

C.A.R. Hoare

There is an unexamined axiom in the “simple and robust” approach similar to that

in academic approaches, the optimization of a small number of variables. Do we really

need all process plants to be operable by the general public? Or are we shooting for a

simpler design than is necessary if we blindly apply this approach?

A typical engineer, Koolen proposes to quantify simplicity with a view to allowing

it to be systematically reduced. Do we really need to quantify simplicity? I have

designed a few small package process plants which are to be operated by the general

public, and I did not apply Koolen’s mathematical/theoretical approach. I knows

simple when I sees it.

It seems that perfection is reached not when there is nothing left to add, but when there isnothing left to take away.

Antoine de Saint Exupéry

Lessons from the slide ruleBefore computers or even electronic calculators, engineers had slide rules. They

couldn’t easily add and subtract and had to guess where the decimal point was. This

meant that engineers needed to be quite adept at mental arithmetic, and only worked

to three significant figures.

My students get nervous when I round things up, do rough sums in my head and

so on, like engineers of my generation. They believe that all 10 of the figures on their

calculator displays are significant, even when I have set them a problem with two sig-

nificant figures in the question data.

Process plant design engineers are probably kidding themselves if they think that

they are working beyond three significant figures. Their underlying data is probably at

best to this degree of precision. The extra decimal places on calculator and computer

screen are spurious precision.

A feel for the sensibleness and reliability of our numbers is what really matters.

Our modern tools seem to be taking this judgment away from new engineers.


Perhaps universities need to start using the QAMA calculators you can get now that

require you to estimate the answer before they will give it to you.

Estimation/feelHe . . . insists that no mathematical formula, however exact it may appear to be, can be ofgreater accuracy than the assumptions on which it is based, and he draws the conclusionthat experience still remains the great teacher and final judge.

James Kip Finch

A feel for the potential error associated with your answer, and its consequent

meaningful precision is very important in engineering (even those who are not

Finch’s assumed “he”s). It is related to the margin of safety and turndown required to

make a plant which will work.

New Scientist magazine conveniently gave us a word for this discipline, “olfactor-

ithmetic,” or the ability to notice if a number “smells” wrong. It will come with

practice, but you can start to get it by always remembering the compounded uncer-

tainty of your sources of data, the imprecision of your heuristics, and the probability

of error.

As a rule of thumb, remember that every engineering design method is based on

assumptions and simplifications; your original design data has an associated degree of

uncertainty, as does any chemical/physical data you are using. If you have not identi-

fied, verified, quantified, and multiplied these assumptions, simplifications and degrees

of uncertainty, your calculations are at best very rough.

There is nothing wrong with rough calculations, as long as they are combined

with professional judgment. If I have sized a piece of equipment using three indepen-

dent rough sizing methods which have a decent track record of success in professional

use, and used professional judgment to make sense of the answers, I am far happier

that I have a robust solution than if I had commissioned a five-year bench-scale

research study.

So, avoid spurious precision—be honest with yourself about how much you really

know, and specify equipment with a margin of safety plus turndown.

SETTING THE DESIGN ENVELOPE

We might call much of what is taught in university “Training Wheels Design.” Just as

we teach kids to ride a bike by minimizing the complexity of the task with a couple

of ancillary wheels, we minimize the complexity of the design task for beginners with

an assumption of “steady state”—all parameters (flow, composition, temperature, pres-

sure, etc.) are assumed to be constant during the life of the equipment.

A real plant operates for most of its life within bounds set by the effectiveness of

its control system, a significant part of its life in commissioning/maintenance


conditions outside these bounds, and has to be sufficiently safe when operating well

out of bounds in an emergency situation.

Although a plant will spend most of its life operating within bounds (though not

actually at steady state), the maintenance and emergency conditions we need to

accommodate may well have a larger effect on the limits of plant design than the

requirements of quasi-steady state normal operation.

So we have to design a plant which will handle normal variations for extended

periods of time, maintenance conditions for shorter periods of time, and extreme

conditions for short, but crucial periods.

Each parameter therefore has a range of values, rather than the single value it has

in the steady state case. So real plant design may have dozens of parameters at each

stage, each of which has a range of values. The range of values will be associated with

a range of probabilities, similar to a confidence interval. If our design is good, extreme

values will be experienced with a low probability, and average values will be

commonplace.

We frequently use confidence intervals to decide on the upper and lower bounds

of incoming and outgoing concentrations of key chemicals. Performance trials are fre-

quently statistically based, so we are in effect already working to a confidence interval

in our product specification. We do not, however, design to the specified confidence

interval: many engineers usually go up at least one standard deviation, such that a 95%

confidence interval specification from the client leads me to work to a 99% confi-

dence interval design.

We then need to permutate these ranges of parameters to generate design cases, repre-

senting the best, average, and worst cases we can imagine across important permutations.

For example, if I am designing a sewage treatment works, it needs to work in the mid-

dle of the night when no one is flushing a toilet and most factories are closed, as well as at

peak loadings. It needs to work during dry weather, and during a 100-year return period

storm. It needs to work sufficiently well when crucial equipment has failed.

When it rains hard, we can initially get a sharp increase in solids and biological

material coming in through sewer flushing, and afterwards we get large volumes of

weak sewage, mostly rainwater.

So I need to design to a low probability/high strength/high flow scenario, a

medium probability/high flow/low strength scenario, a medium probability/low

flow/high strength scenario, and a high probability/medium flow/strength condition.

I can imagine those who think chemical engineering to be petrochemical engi-

neering thinking this only applies to water and sewage treatment, but this is just a

specific example of a general condition: oil is a natural product too, whose production

is subject to wide variations in flow and composition.

In offshore oil and gas design only a small number of test/appraisal wells are

drilled, and good engineering judgment has to be applied to evaluate the impact of


the uncertainty of the data obtained from well fluid analysis on production, operation,

and flow assurance.

Even bought-in chemicals have a specified range of properties, rather than a single

value. There is in short no such thing as steady state.

We need to construct a range of realistic design scenarios, and our designs need to

work in all of them. It may be that our provision for less probable scenarios has a

shorter service life or requires more operator attention than that for the normal run-

ning case, but the plant has to be safe and operable under all foreseeable conditions.

Summary statisticsIn real design scenarios you often have too little data to generate statistically significant

design limits. We get a feel for the data by generating summary statistics; means, max-

ima, minima, confidence intervals, and so on.

Excel used to come with a plugin which helpfully generated this set of stats but now

you need to plug in the formulae and functions yourself, although it doesn’t take very long.

The lack of a formally statistically valid data set doesn’t usually mean you get out

of designing the plant. Sensitivity analysis can be used to see how much it matters if

your data is unrepresentative.

The all-too-likely lack of rigorously valid data as a design basis should be some-

thing the designer is conscious of throughout the design. Those who claim that arro-

gance in process design can usefully be measured in nanomorans may be surprised to

see me use the word, but we need to demonstrate humility.

For example, the well samples discussed in the last section are limited in number,

and will not be fully representative for many reasons. Types of sampling methods, res-

ervoir condition, insufficient well conditioning prior to sampling collection, inappro-

priate sampling collection methods, limitation of laboratory testing, contamination of

sample by drilling mud, methanol, and so on may contribute to error. Different labo-

ratories have been shown to produce wide inconsistency in compositional analysis

even for the same sample.

All these uncertainties may affect the surface facility design further downstream

such as pipelines and onshore terminals. Among the examples of impacts are incorrect

sizing of separator, compressor, pumps; over-prediction or under-prediction of liquid

holdup in pipelines, slug catcher capacity, fuel gas systems, and so on.

It is therefore important that the design parameters are not too tight and that a suf-

ficient design margin is provided.

IMPLICATIONS OF NEW DESIGN TOOLS

Computer-based tools allow you to do more brute force calculations, so you don’t

have to design a plant in such a way as to make analysis simple. You should not,


however, take advantage of this capability—your professional responsibility is to

understand your plant. You also need to be careful not to be carried away by the pre-

cise looking outputs of these programs.

You should not use modeling and simulation programs as a substitute for design—

by the time you have taken your usually rather flimsy design data and run it through a

black box program written by someone who has never designed a plant, outputs are

at best merely informative, and can easily be highly misleading.

There is more specific comment on these issues elsewhere in the book, but many

of the things you were taught to do in university are not really design, and many of

the tools you used are not used by professionals, with good reason.

IMPORTANCE OF UNDERSTANDING YOUR DESIGN

Recent stock market crashes have been at least contributed to by automated stock market

modeling software, in many cases written by physical scientists and engineers based on

models derived from the physical sciences very similar to process simulation software.

Your calculations, whether they be done by hand, in a spreadsheet, or a simulation

program, are a model of the proposed system. It is infinitely better to have a simpler model

which you understand well than a more complex one which you don’t really understand.

In Henry Petroski’s books, many of his engineering disasters come about as a result of

people who thought they understood well-established design methodologies cutting

safety margins and applying techniques to areas where their underpinning assumptions

did not hold.

My advice is not to take responsibility for anything you don’t understand well

enough. This can make you unpopular if there is a lot of time pressure to get calcula-

tions signed off, but engineering is hard, and the stakes are high.

There’s no shame in asking someone to show you why they think they have a

design nailed down if you can’t see how they have, and our professional responsibility

means we have to imagine defending our actions in court later. Don’t allow yourself

to be pushed around by management. Their aims may be different from yours, which

leads to a necessary tension in design.

MANAGER/ENGINEER TENSIONS IN DESIGN

Managers and engineers have to some extent different pressures upon them and different

aims to meet during the design process, and the inevitable tension arising from this

needs to be managed. An old engineering joke illustrates the differences in outlook:

A man in a hot air balloon realized he was lost. He reduced altitude and spotted a manbelow. He descended a bit more and shouted, “Excuse me, can you help me? I promised afriend I would meet him half an hour ago, but I don’t know where I am.”


The man below replied, “You are in a hot air balloon hovering approximately 30 feetabout the ground. You are at approximately 53 degrees north latitude, and at 1 degree 13minutes west longitude from the Greenwich meridian.”

“You must be an engineer,” said the balloonist.“I am,” replied the man, “but how did you know?”“Well,” answered the balloonist, “everything you told me is technically correct, but I have

no idea what to make of your information, and the fact is I am still lost.”The man below responded, “You must be a manager.”“I am,” replied the balloonist, “how did you know?”“Well,” said the man, “you don’t know where you are or where you are going. You made a

promise which you have no idea how to keep, and you expect me to solve your problem. Thefact is you are exactly in the same position you were in before we met, but now, somehow,it’s my fault.”

Manager/engineer tensions I: risk aversionThere is a tension between external design consultants and product managers in client

organizations, but even when designer and product manager are within the same

organization, there may be a tension. Designers are usually risk averse, and manage-

ment are often more risk tolerant.

In essence, management usually want to get a product to market as soon as possi-

ble, and with the absolute minimum possible margins of safety, and highest possible

profit margin. Designers don’t want to design things which don’t work, they have a

good idea of the limits of their analytical techniques, and they know that all design

relies on approximation and heuristics.

When nondesigners look at our calculations and drawings, it all looks very mathe-

matical, very sharp-edged, but these are precise-looking calculations of approximate

values, and those straight lines on the drawing might be different in ten thousand ways.

I hear that management are now asking engineers to justify adding any margins of

safety at all to designs done substantially using modeling programs, but that engineers

are then being asked to carry out debottlenecking of plants which have been designed

in this way.

So there will always be a tension between engineers and management. The Scotty

Principle addresses this tension with respect to timescales:

Scotty Principle (n.) The de facto gold star standard for delivering products and/or serviceswithin a projected timeframe. Derived from the original Star Trek series wherein Lt. Cmdr.Montgomery “Scotty” Scott consistently made the seemingly impossible happen just in time tosave the crew of the Enterprise from disaster.1. Calculate average required time for completion of given task.2. Depending on importance of task, add 25�50% additional time to original estimate.3. Report and commit to inflated time estimate with superiors, clients, etc.4. Under optimal conditions the task is completed closer to the original time estimate vs. the

inflated delivery time expected by those waiting.Urban Dictionary


However, margins of safety are not just about gaining a Scotty-style reputation as a

miracle worker. When things are uncertain, designers should err on the side of cau-

tion, “under-promise and over-deliver.” Occasionally you will lose out to those who

are willing to gamble, but winning in that way makes them lucky, not good. Let’s not

trust to luck when so much is at stake.

Manager/engineer tensions II: the Iron TriangleOne other point of potential conflict between designers and management is covered by a

trilemma sometimes known as the Iron Triangle; “Fast, Good, or Cheap—pick any two.”

It is common in my experience for managers and clients to want three out of

three, but it just can’t be done. Scotty knew about this too: “I cannae change the laws of

physics, Jim”—neither can we change the iron laws of design.

WHOLE-SYSTEM DESIGN METHODOLOGY

Our initial conceptual design process needs to do more than just select broad technol-

ogies—such a decision making exercise on its own isn’t really engineering design at

all. A conceptual design needs to give us an approximate size and shape of unit opera-

tions and associated pipes, pumps, and so on.

They need to allow us to price at least all Main Plant Items (a term used in

approximate costing functionally close to identical with Unit Operations). We need to

lay all the kit out on a General Arrangement (GA) drawing to make sure it will fit on

the site available. Ideally we would have some idea of how long it will take to build,

and what it will cost to run it.

The sizing of unit operations for this purpose will be based on easy rules of thumb.

We only need to know roughly how big they are. We will need to do a rough mass

and possibly energy balance to allow us to size the units, as recycles can make flows

far larger than are obvious by simple inspection.

We cannot optimize this rough design, and we should not try to. Conceptual

design should under-promise and over-deliver. We should make very conservative

assumptions, and err consistently on the side of caution. Things always take longer

than you think, cost more, and have problems which are not immediately apparent.

Save a bit of fat for later—you’ll need it.

Detailed designs will need to address a number of issues which were

disclosed but not resolved by the conceptual design stage. Rather than looking to

scientific literature or modeling programs for answers, professional process plant

designers usually look to the know-how of experienced engineers and equipment

suppliers.

They may need to carry out slightly more rigorous design than at the conceptual

stage to do this, and they will certainly need to get their P&ID, mass and energy


balances tightened up considerably to do so. Again, this tightening-up should be based

upon commercially available kit.

At this stage (if not earlier), the designer will need to supply the other engineering

disciplines involved with the information they require to carry out their design.

Civil engineering designers will need to be provided with equipment weights,

locations, and so on. Electrical engineers will need motor sizes and preferred starter

types. Software engineers will need a control philosophy and a P&ID. They will all

usually be in a hurry to get this information, but they will complain if it changes too

many times during the design process.

The other disciplines (and sometimes the client) will come back with suggestions

for how the overall design might be changed in ways which make it better, cheaper,

and easier to build (or whatever). The plant designer needs to evaluate, negotiate and,

fairly frequently, modify their process design to accommodate these changes.

Design for construction has to specify every detail, right down to the numbers and

types of bolts used to connect pipe flanges. This stage is not merely the dull schedul-

ing and documentation stage envisaged by theorists. Many design challenges remain—

the devil is in the detail.

DESIGN STAGES IN A NUTSHELL

These are generalized consensus practice; additional disciplines and deliverables feature

in certain sectors—see “Variations on a theme” to follow.

Conceptual design• Construct Design Envelope.

• Produce Process Flow Diagram (PFD), P&ID, GA, unit op design, mass and

energy balance, rough costing, and preliminary safety study.

Purpose: To identify the design philosophy, the most promising technologies,

rough cost, and footprint, then select the one or two most promising technologies.

See the conceptual design section of later chapters for details of safety, costing, lay-

out, materials and equipment selection, process control, and hydraulics aspects.

Intermediate design• Produce more detailed PFD, P&ID, GA, unit operation design, mass and energy

balance, control philosophy, detailed costing, robust safety study, obtain quotations

and specifications from suppliers for equipment, and have mechanical, civil, con-

trol, and electrical design progressed in tandem.


Purpose: Choose sub-technologies, design approaches produce good quality cost-

ing and layout to allow evaluation and quite possibly to place an order for construc-

tion for long lead time items such as compressors.

See the intermediate design section of later chapters for details of safety, costing; layout,

materials and equipment selection, process control, hydraulics, and optimization aspects.

Design for construction• To produce the PFD, P&ID, GA, unit operation design, mass and energy balance,

control philosophy, detailed costing, robust safety study, datasheets, schedules,

obtain exact specifications and prices for equipment to be purchased, finalize tech-

nical bid evaluation, and have mechanical, civil, control, and electrical design pro-

gressed in tandem.

Purpose: Design of plant to be built.

VARIATIONS ON A THEME

I was going to include here a big diagram showing all the stages of an ideal design

methodology, but that might give you the impression there was such a thing. The

truth is that the contents of the preceding chapters represent a core common

approach, but that there are great variations between sectors, between companies, and

between countries in a number of key issues, even if we stick to the whole-plant

“grassroots” design which this book is about.

Operating companies, contracting/Engineering, Procurement and Construction

(EPC) companies, and consultants of various kinds will all do a thing which they call

design. The meaning of the term will vary, as will their understanding of terms like

conceptual design, detailed design, and so on.

The job titles of the people involved in design, the tools they use, and the deliver-

ables they produce will differ. There are many additional sector-specific deliverables I

have not covered here, in the interests of simplicity. There are also sector-specific roles

such as the petrochemical industry’s control/instrument engineer.

The contents of the preceding chapters are true to the best of my knowledge for

all sectors, as far as they go. There is, however, an infinite amount of detail which I

have “omitted for clarity” as we engineers say.

FURTHER READINGAnon, 2012. Specification for Unfired, Fusion Welded Pressure Vessels. PD5500 British Standards

Institute.Anon, 2002. BS EN 13445: European Standard for Unfired Pressure Vessels. British Standards Institute,

London.Koolen, J.L.A., 2001. Design of Simple and Robust Process Plants. Wiley-VCH, Weinheim.





CHAPTER 8

How to Do a Mass and Energy Balance

INTRODUCTION

Most universities teach students to carry out mass and energy balances in single steady

state scenarios in order to simplify the process for beginners. In the real world there

is, however, no such thing as steady state.

Real plants are dynamic, with variable flows, compositions, temperatures, pres-

sures, and so on. They have to produce product(s) to specification under all of these

conditions, though it should be noted that “to specification” also implies a range of

acceptable compositions rather than a single one.

Producing a model which accurately and dynamically models the operation of a

plant which has not yet been constructed is impossible. We can, however, produce a

number of steady state models across the design envelope, which allow us to generate

performance curves and sensitivity analyses which give us confidence that the design

will work (where the meaning of “work” is not perfection, but that implied by the

brief) across all reasonably foreseeable conditions.

We need to exercise engineering judgment in deciding which scenarios we need

to consider. In designing a water treatment plant we might, for example, look as a

minimum at high flow and low flow scenarios, permutated with high contaminant

and low contaminant levels.

We would also consider scenarios in which key process equipment is out of service

for backwashing or maintenance, variations in feed temperature, degradation in dosed

chemical quality, and so on.

This is almost always done in practice using a spreadsheet program, and the

required permutation of scenarios becomes quite easy to achieve (and just as impor-

tantly, verify) if we structure our spreadsheet in a certain way.


Unsteady stateIf you have an opportunity to view the “trends” screens on a supervisory control and

data acquisition (SCADA) system, the dynamic nature of plant operation becomes

very obvious (Figure 8.1).

Everything varies with respect to time on a process plant at some scale of resolution.

Feedstock quality varies, as does product quality. Plant throughput varies both because we

call for a different flow rate from the system, and because equipment degrades over service

intervals, so that the same control inputs may produce different responses over time.

Other parameters vary for similar or different reasons. When we take off our steady

state “training wheels,” we need to understand the range of possible values which we

can encounter in all parameters. A robust design works under all of these conditions.

It is often the case that the range of parameters we consider has an associated prob-

ability. If we have been provided with data which we are to use as a basis for design,

the upper and lower limits of confidence intervals may well carry more weight than

the maximum and minimum figures in the data.

So we are (consciously or not) designing a plant that will only probably work.

Usually the required probability of success is implied by the performance tests we

have to pass to have the plant accepted (though our design’s probability of success

should always be higher than that required by the test).

Figure 8.1 Unsteady state SCADA screen. Image reproduced courtesy of the Process EngineeringGroup, SLR Consulting Ltd.


Implications of feedstock and product specificationsMost of things I have designed which have actually been built have been water and

effluent treatment plants. The feedstock for such plants is a very variable flow of water

with very variable levels of contamination. Drinking water plants ramp up and down

production to match demand. Sewage and industrial effluent treatment plants have to

reliably treat the often highly variable flows and compositions they are given.

Drinking water has to meet very tight and absolute maximum allowable value

specifications for more than a hundred parameters. Effluent treatment product specifi-

cations are less complex, and may be probabilistic—95% compliance with specification

might be fine.

What is true in water is true in all sectors: a plant needs to handle the worst feed-

stock it might encounter as well as the specification says it has to, producing the prod-

uct to the specification required as reliably as is required.

There is no benefit in exceeding the plant’s required performance, and it costs

money to do so, but it has to meet required performance as specified, or for practical

reasons a little better.

Stages of plant lifeWhen setting out our design scenarios, we need to consider all stages of the plant’s life.

This is especially important if we have attempted to design in integration of systems.

If there is a stage of the plant’s life, such as commissioning, start-up, shutdown, or main-

tenance, when we will need to run the plant in a different way, or need additional services,

the full implications of this need to be considered in our mass and energy balance.

In the petrochemical industry, they mount campaigns called turnarounds where

staff carry out more or less all the maintenance for a plant in a short sharp program. Is

this how your plant will be maintained, or will it be little and often? The designer

needs to know, and to take this into consideration from the stage of initial mass bal-

ance generation.

HANDLING RECYCLES

It is frequently the case that there are process streams on a plant which are returned

directly or indirectly to the feed of the unit operation which they came from, and

these are known generically as recycles.

As their introduction modifies the stream going into the unit operation, and the

product of an operation is almost always affected by its feed stream, recycles affect

themselves and, having been affected, affect themselves further, and so on. This may

not be obvious during “steady state” operation, but it will be obvious to the designer.

Back when I was a student, we used to have to resolve this issue using iterative cal-

culations done by hand. This was very dull indeed, the time-consuming nature of the

approach meaning that nested recycles (recycles within recycles) were avoided.

111How to Do a Mass and Energy Balance

Now we have MS Excel, which offers us a number of ways to carry out iterative

calculations in seconds, and without the mistakes borne out of loss of attention which

used to appear in hand calculations.

HOW TO SET IT OUT IN EXCEL

Here is how I set it out, which is pretty similar to the way most other professionals

do. You don’t have to do it the way I do, but you do need to address the problems

I have done at least as robustly as my method does. Much of what professional engi-

neers do habitually is intended to systematically avoid error, and make it easy to spot

any residual errors.

Mass and energy balances like these are pretty complicated process models, which

are very hard to hold in your head as a completely unified whole. Even the best

examples will need rigorous double-checking by a second competent engineer, and

you should assume that there will be mistakes before such checking.

I like to make my Excel spreadsheet look like the engineer’s calculation pads we

used to use back when we did hand calculations, and I teach my students to do this as

well. If I do not do this, I get given spreadsheets which are hard to follow, with bits

of calculation over in some obscure corner of the spreadsheet which no one notices.

Forcing the calculations into a succession of virtual sequential A4 sheets makes it

easy to set out an annotated logical argument, and to follow the argument being made.

So I recommend a vertical stack of these virtual pages be set out on each Excel tab.

I start with a header tab which sets out the given and assumed design parameters

and gives an overview of the whole spreadsheet. All uses of these design parameters

throughout the spreadsheet should be linked directly back to this header page. This

makes it easy to vary the parameters to generate different scenarios.

If the cells containing the parameters have been labeled with descriptive names,

the designer and the checker will not need to keep flicking back to the header to see

what a parameter is when it is encountered on other sheets. I would in fact recom-

mend that all cells whose values are copied across into calculations are so labeled to

reduce errors and facilitate checking.

Each subsequent tab has a stack of pages which represent the design of a unit oper-

ation. I like to follow the order of the process flow on the Piping and Instrumentation

Diagram (P&ID) in my virtual stack of Excel tabs. The mass and energy flows out of

each unit operation should be (with the exception of those requiring the breaking of

a recycle to avoid circular calculation errors—see later) directly linked to the inputs to

the next operation.

This makes the spreadsheet capable of self-modifying to handle the various scenar-

ios which it will be used to balance, removing the possibility that the designer will

forget to change cells.


Each of these tabs will have what I call “checksums”, calculations arranged so that

they will be zero if the unit operation’s mass balance is correct. These checksums will

be carried across to the header tab, where they will appear as a single table, along

with checksums for mass and energy balance checks at scales above the single unit

operation.

Behind these mass and energy balance/unit operation sizing tabs I would usually

have a few sheets of hydraulic calculations, so that I can include dynamic pump calcu-

lations based on the flows from the mass and energy balance in the spreadsheet for

convenience. I have standard single-tab Excel spreadsheets (verified by an independent

third party and then locked) which I insert for this purpose.

I also have such standard spreadsheet tabs for the more common unit operations

which I design, saving time and reducing errors. Going to the trouble of producing

such standard spreadsheets and having them validated is, however, only worthwhile if

you are going to use them many times.

USING EXCEL FOR ITERATIVE CALCULATIONS: “GOAL SEEK”AND “SOLVER”

Microsoft Excel has a suite of commands known as “what-if analysis” tools. Process

plant designers find two of these particularly helpful: Goal Seek and Solver.

Goal Seek allows us to vary the number in a spreadsheet cell until the value of a

cell whose contents are calculated from the first cell’s value is a number we specify.

Solver is more sophisticated, and allows us to minimize or maximize a value in a

target cell.

These are very handy to plant designers—I use Goal Seek for the following

purposes:

• Resolving recycles in mass and energy balances—use Goal Seek to make the

difference between two mass balance formulae equal zero, and you have resolved

the recycle;

• Dealing with iterative calculations, especially those with too many unknowns—I

use this a lot for hydraulic calculations based on the Darcy�Weisbach equation

and Colebrook�White approximation, as described in Chapter 9.

Solver, on the other hand, is well suited for optimization exercises such as sensitiv-

ity analysis and reactor design problems.

It may be useful to break the chain of calculations in nested recycle calculations

with a cell whose value can be set manually, prior to the “what-if ” command being

run. This avoids “circular argument” error messages, but great care should be taken

that all results yielded by such an exercise are sensible, as there is a greater probability

of nonsensical results from this technique.

113How to Do a Mass and Energy Balance

CHAPTER 9

How to Do Hydraulic CalculationsINTRODUCTION

I am an old man now, and when I die and go to Heaven there are two matters on which Ihope for enlightenment. One is quantum electrodynamics and the other is the turbulentmotion of fluids. And about the former I am rather more optimistic.

Sir Horace Lamb

At university, all chemical engineers study fluid mechanics, which is a kind of

applied mathematics, usually combined with a bit of applied dishonesty.

The truth is that no one really understands the turbulent motion of fluids, or can

predict it with a high degree of precision. Consequently, even the most basic fluid

mechanics courses have to handle a transition from first principles mathematics to the

rough heuristic of the Moody diagram.

This is the point where an honest lecturer should admit we can’t actually use

Bernoulli’s equation to solve any useful problems, and we are consequently bringing

in a chart based on empirical relationships determined by experiment to fill the gaps

in our understanding with a fiddle factor.

Not all lecturers are so honest or insightful and students may leave university

thinking that they understand something which no one does—until someone asks

them to size a pump.

Hydraulics is the more practical cousin of fluid mechanics, which we mainly use

to specify pump and equipment sizes as accurately as required for practical purposes.

Engineers don’t have to completely understand things in order to exercise sufficient

control to achieve a given aim.


Right at the start of a design project, we need to know, for example, whether we

intend to move fluids around our plant by gravity, or by pumping. Even to do the

most basic layout we need to know approximate pipe diameters.

The nature of hydraulic calculations is essentially iterative. We need to have a pipe

diameter and pipe length to work out the headloss down a pipe. We need to know the

headloss to know the economic pipe diameter. We need to know static and dynamic

head to know how big our pump needs to be. We need to know how physically large

our pump is to know how long the pipe needs to be.


We have to start this circular process somewhere. The method used by most process

engineers is described in the following sections. Levels 1/2 are used at conceptual design

stage, Level 2/3 at detailed design. Level 4 is used only in rare cases of complexity.

Hydraulic calculations have three main components: static head (the elevation

from reservoir to point of discharge, plus any atmospheric pressure difference between

reservoir and point of discharge), straight run headloss (headloss due to friction at

operating flowrate due to straight pipe sections), and fittings headloss (headloss due to

friction at operating flowrate due to bends, tees, valves, etc.).

In water process engineering, we make a fair amount of use of open channels, and

I consequently have to do quite a lot of open channel hydraulics. However, I am

going to leave that out of this book, as it can be quite complicated and there isn’t

very much of it in most other process sectors.

I am also going to leave out calculations for multiphase flow and water hammer.

I suggest you look at Donald Woods’ book (see Further Reading) for guidance on

shortcut methods for these though, in practice, you might want to call in a specialist

if your rough calculations make you think these conditions likely.

Level 1—superficial velocitySuperficial velocity is the same thing as average velocity (i.e., the volumetric flowrate

in m3/s divided by the pipe’s internal cross-sectional area in m2—its units are in this

example m/s).

A very quick way of starting our hydraulic calculations is to use the following rules

of thumb from acceptable superficial velocities:

• Pumped water-like fluids ,1.5 m/s

• Gravity fed water-like fluids ,1 m/s

• Water-like fluids with settleable solids .1, ,1.5 m/s

• Air-like gases 20 m/s

Two-phase flow is hard to predict, and should be designed out if at all possible—

headlosses can be one thousand times that for single phase flow.

These rules will usually give sensible headlosses for the sort of pipe lengths

normally found on process plants.

Level 2—nomograms, etc.The most difficult part of a headloss calculation is determining the straight run head-

loss. It isn’t really that difficult, but we have to do it many times, so a quick method is

handy at earlier stages of the design.


LiquidsPipe manufacturers and others produce tables and nomograms which can be used to

quickly look up headloss due to friction for liquids (Figure 9.1).

Figure 9.1 Pipe flow chart nomogram. Copyright material reproduced from Sandler, H.J. andLuckiewicz, E.T. (1987) Practical Process Engineering: A Working Approach to Plant Design.

117How to Do Hydraulic Calculations

We can then calculate fittings headloss by the k value or equivalent diameter

method (obtaining a count of valves etc. from the Piping and Instrumentation

Diagram (P&ID), and bends, tees, and so on from the General Arrangement (GA)

drawing), and work out the static head from heights measurable from our GA,

plus vessel pressures read from our Process Flow Diagram (PFD). This is one

of the reasons why even quite early stage designs need to produce all three of

these drawings.

It may be seen that once we have carried out the hydraulic calculations, our pump

and possibly pipe sizes will need to change, as might minimum and maximum operat-

ing pressures at certain points in the system. There might even be a requirement to

change from one pump type to another, or to change from a fan to a blower or from

a blower to a compressor.

So there is a stage of design development which takes a set of preliminary drawings

and modifies them to match likely hydraulic conditions across the design envelope.

This stage requires us to do lots of approximate hydraulic calculations before the

design has settled into a plausible form.

We consequently do the quickest and the least rigorous calculations which meet

the needs of this stage of design development as described in this section.

Net positive suction headAt Level 2, I would also recommend calculating net positive suction head (NPSH), as

it can affect a lot more than just pump specification. There is a good description of

what this is and how to do it in Coulson and Richardson Volume 1. I recommend

producing an Excel spreadsheet based on this approach, using the Antoine equation to

estimate vapor pressures.

Note that NPSH is calculated differently for centrifugal and positive displacement

pumps, varying with pump speed for positive displacement (PD) rather than pressure

as with centrifugal.

GasesIf we are working with an air-like gas, we can use the charts of friction losses in ducts

for air which are readily available to estimate straight run headloss (Figure 9.2).


If headloss due to friction is ,40% of upstream pressure (as it usually is), we can

ignore compressibility effects for gases at Level 2, and use the same method as sug-

gested for liquids above.

Level 3 (now superseded)—Moody diagramStudents are mostly taught to calculate straight run headloss using a Moody diagram,

which is a summary of empirical experiments (and essentially an admission of defeat on

the part of the mathematicians and scientists responsible for fluid mechanics—they

couldn’t make their sums work without these fiddle factors taken from experimental data).

The Moody diagram is one of the things superseded by MS Excel. As Excel can’t read

charts, we use curve-fitting equations which approximate the Moody diagram’s output.

While this is an approximation, it might well be closer to the true experimental

value than is read by the average person from an A4 copy of a Moody chart. In any

case, it’s a fiddle factor.

Figure 9.2 Gases: duct chart. Copyright material reproduced courtesy of www.engineeringtoolbox.com.


http://www.engineeringtoolbox.com

Level 3 (updated): spreadsheet methodLiquidsI personally use the Colebrook�White approximation to give me the fiddle factor

which I would once have read from the Moody diagram, and I plug this into the

Darcy�Wiesbach equation to work out straight run headloss, with an iterative

method based on Excel’s Goal Seek function which I cover in Chapter 8.

I recently read a paper (see Further Reading) which suggested there are new and

more accurate curve fitting equations, and I might have got around to modifying

my standard hydraulic calculation spreadsheet if I hadn’t gone to all the trouble of

having it validated.

So if you are producing your own spreadsheet for this purpose, I suggest you look

into the Zigrang and Sylvester or Haaland’s equations, which this paper recommends,

to generate your fiddle factor.

This approach allows you to calculate straight run headloss to the degree of

accuracy required for more or less any practical application.

Static head and fittings headloss can then be calculated as in Level 2, and it can all

be added up to generate a delivery side headloss.

Suction side headloss and NPSH should also be calculated, and all of this used

to generate an approximate pump power rating for a centrifugal pump using the equation:

P5Qρgh= 3:63 106� �

η

where

P5 power (kW)

Q5 flowrate (m3/h)

ρ5 density of fluid (kg/m3)

g5 acceleration due to gravity (9.81 m/s2)

h5 total pump head (m of fluid)

η5 pump efficiency (allow 0.7 if you don’t have a figure).

The manufacturer will give you the precise power ratings and motor size, but the

electrical engineers will need an approximate value of this (and pump location) quite

early on in the design process, to allow them to size their power cables. You should

err on the side of caution in this rating calculation, as the electrical engineers will be a

lot happier with you if you come back later to ask for a lower power rating than if

you ask for a higher one.

GasesCompressibility can make this all get a bit complex, but we can simplify matters.

Crane, the valve manufacturers, proposed a simplified method in a technical paper first

published in 1942 (see Further Reading).

If headloss is ,40% of upstream pressure (as it usually is), gas compressibility can

be ignored, and we can use the Darcy�Weisbach equation/k-value method to


determine headloss. The gas density used should be consistently either that upstream

or downstream for headloss ,10% of upstream pressure.

For headloss of 10�40% of upstream pressure, use the density at the average of

upstream and downstream conditions. If it is .40% of upstream headloss, we will

need to consider compressibility and use the Weymouth, Panhandle A, and Panhandle

B equations. It should be clear that this will require an iterative design process.

Level 4—CFDI have never had to carry out a Computational Fluid Dynamics (CFD) study, though

I know other professional engineers who have, so it isn’t ridiculously theoretical

and impractical.

It is, however, rare enough that it is more likely that you will go out to a specialist

subcontractor to do it for you rather than buy the software and learn to use and

validate it for a one-off exercise.

HYDRAULIC NETWORKS

The previous section is about how to calculate the headloss through a single line, but

what about the common situation where we have branched lines, manifolds, and so on?

Each branch is going to take a flow proportional to its headloss, and its headloss

will be proportional to its flow. Producing an accurate model can become complex

very quickly. Things which are at all hard to model/understand are generally not

robust design unless this represents the only workable approach.

My approach to this is to reduce complexity and improve design as follows:

• Avoid arrangements of manifolds which give a straight-through path from feed

line to branch. Entry perpendicular to branch direction is preferred.

• Oversize manifolds such that superficial velocity never exceeds 1 m/s at the highest

anticipated flowrate.

• Step the manifold diameter along its length to accommodate lesser flows to further

branches.

• Include a small hydraulic restriction such that branch headloss is 10�100 times

that from one end of the manifold to another.

• Design in passive flow equalization throughout the piping system wherever possi-

ble by making branches hydraulically equivalent.

I then do headloss calculations for each section of the plant at expected flows to

find the highest headloss flow path through my simplified design.

I use this path to work out the required pump duty. I will test it at both average

flow with working flow equalization, and at full flow through a single branch. Usually

these don’t differ all that much, and as I know that the more rigorous answer

lies between them, I don’t worry about it. Only if the results of this approach seem

problematic will I do a more rigorous analysis.


To do a more rigorous analysis, I create an Excel spreadsheet based on the Hardy

Cross method to solve for individual pipe flows. The “Solver” function can be used

to find the change in flow which gives zero loop headloss.

There are many computer programs available to do these calculations, but I would

personally always rather produce a simple model in MS Excel—which I completely

understand—than use black box programs.

PUMP CURVES

A notable omission from university courses is an understanding of how to read a

pump curve, which is an essential requirement to do what we are probably going to

do with the head/flow pairs we calculated across the design envelope.

The most frequent use of pump curves is for the selection of centrifugal pumps, as

the flow rate of these varies so dramatically with system pressure. Pump curves are

used far less frequently for positive displacement pumps.

A basic pump curve plots the relationship between head and flow for a pump at a

given supply frequency. On more sophisticated curves, there may be nested curves

representing the flow/head relationship at different supply frequencies or rotational

speeds, with different impellers, or different fluid densities. The pattern is that curves

for larger impellers or faster rotation lie above smaller impellers or slower rotation,

and lower specific gravity above high for centrifugal pumps.

Let’s start with a basic curve (Figure 9.3):

Head unitsH (m)

p (kPa)

Performancecurve

PressurePSI

System characteristic

Flow units(m3/h)(I/s)

Figure 9.3 Basic pump curve. Copyright image reproduced courtesy of Grundfos.


Along the horizontal axis we have increasing flow (Q), and along the vertical axis,

increasing pressure (H). The curve shows the measured relationship between these

variables, so it is sometimes called a Q/H curve. The intersection of the curve with the

vertical axis represents the closed valve head of the pump. These pumps are generated

under shop conditions and ideally represent average values for a representative sample

of pumps.

We can use our calculated flow/head pairs to plot a system head on the same

axes, and see where our system head meets the Q/H curve. This will represent the

operating or duty point of the pump.

We will have a system head curve for the expected range of flows at a given system

configuration. Throttling the system will give a different system curve. We will need

to produce a set of curves which represent expected operating conditions, generating

a set of duty points.

That’s it as far as our basic curve is concerned, but it is common to have efficiency

and motor rating curves plotted on the same graph (but not the same vertical axes) as

in the example in Figure 9.4.

H h[m] [%]

50

70

60

50

40

30

20

10

0

12

10

8

6

4

2

40

30

20

10

10

8

6

4

2

0

00

P2

[kW]

10 20 30 40 50 60 70 NPSH(m)

Q [m3/h]

Efficiency

Power consumption

NPSH

Figure 9.4 Intermediate pump curve. Copyright image reproduced courtesy of Grundfos.


So we can see that we can draw a line vertically from the duty point to the effi-

ciency curve, and obtain the pump efficiency at the duty point by reading the vertical

axis at the point of intersection. Similarly we can draw a vertical line to the motor

duty curve, and obtain a motor power requirement.

Having tackled these basic and intermediate curves, we can look at the common

format of professional curves, incorporating efficiency, NPSH, and impeller diameters

like this (Figure 9.5).

Figure 9.5 Complex pump curve. Copyright image reproduced courtesy of Grundfos.


These start to look a bit confusing, but the thing to bear in mind is that,

just as with the simpler examples, the common axis is always the horizontal one

of flowrate. So the corresponding value on any curve is vertically above or below the

duty point.

These more advanced curves usually come with efficiency curves, and it is usually

visually obvious that these curves seem to bound a region of highest efficiency. At the

center of this region is the best efficiency point or BEP.

We will want to choose a pump which offers good efficiency across the range of

expected operating conditions. Note that we are not necessarily concerned with the

whole design envelope here—it is not crucial to have high efficiency across all

conceivable conditions, just the normal range.

A well-selected pump will have a BEP close to the duty point. If the duty point is

way over to the right of a pump curve, well away from the BEP, this is not the right

pump for the job. Try another.

These are the basics of centrifugal pump selection. If you are in a position

to influence which pump is purchased, any pump supplier’s representative would

be happy to talk to you about pump selection for as long as you are willing to

listen. Probably buy you lunch too, though obviously that wouldn’t affect your

choices.

Even with the most cooperative pump supplier, the curves you want in order

to make a pump selection may not be available, as is commonly the case when

we want to use an inverter to control pump output by speed. We can, in this case,

generate the required curves for ourselves using pump affinity relationships. The

laws are:

• Flowrate2/Flowrate15Impeller diameter2/Impeller diameter15Pump Speed2/

Pump Speed1• Dynamic Head2/Dynamic Head15(Impeller diameter2/Impeller diameter1)

25(Pump

Speed2/Pump Speed1)2

• Power Rating2/Power Rating15(Impeller diameter2/Impeller diameter1)35(Pump

Speed2/Pump Speed1)3

• NPSH2/NPSH15(Impeller diameter2/Impeller diameter1)x5 (Pump Speed2/

Pump Speed1)y

Where subscript 1 designates an initial condition on a known pump curve, and

subscript 2 is some new condition.

The NPSH relationship is a lot more approximate than the others. x lies in the

range 22.5 to 11.5, and y in 11.5 to 12.5. A worst-case estimate can be established

using the maximum quoted x and y figures if impeller speed or diameter is to be

increased, and the lowest figures if it is to be decreased.


FURTHER READINGAnon, 2009. Flow of Fluids through Valves, Fittings, and Pipe. Technical Paper 401, Crane Co.Cross, H., 1936. Analysis of flow in networks of conduits or conductors. Engineering Experiment

Station Bulletin No. 286.Genic, S., Srbislav Genic, Arandjelovic, I., Kolendic, P., Jaric, M., Budimir, N., Genic, V., 2011.

A review of explicit approximations of Colebrook’s equation. FME Transact. 39, 67�71.Woods, D.R., 2007. Rules of Thumb in Engineering Practice. Wiley-VCH, Weinheim.



PART 3

Low Level DesignEnough background—exactly how do you design like a pro? Some of this section is

the professional version of the subjects which are taught in university, and some will

be completely new or may even contradict what is taught in academia.

CHAPTER 10

How to Design and Select PlantComponents and MaterialsINTRODUCTION

Engineering is the art of modeling materials we do not wholly understand, into shapes wecannot precisely analyze so as to withstand forces we cannot properly assess, in such a waythat the public has no reason to suspect the extent of our ignorance.

A.R. Dykes

The selection of the basic subcomponents of process plants is an essential part of

what plant designers do. There is often a fundamental misunderstanding in academia

of what constitutes the elements of a process plant.

Process plants are not made of ideas, or (at an engineer’s usual resolution of vision)

even of chemicals. Process plants are made of commercially available products.

We are not usually employed to select process chemistry, but to specify the

pumps and valves, pipes and tanks used to construct the plant in which those

chemistries occur.

As a result of the composition of university departments, taught chemical

engineering often overemphasizes science and mathematics to the point where

graduates lack the broad overview of available technologies which allows them to

make such a selection.

Such qualitative knowledge may seem less intellectually rigorous than science and

mathematics, but it is actually far more sophisticated to exercise multidimensional

judgment in a mental space of the qualities of various process options than to grind

through a rote calculation which a computer could beat you at.

In this chapter I will attempt to offer the broad guidance on the selection of

common items which is missing from many chemical engineering programs.

WHAT PROCESS ENGINEERS DESIGN

The essence of process engineering is integration of complex systems, but in order to

integrate systems, the designer has to have some knowledge of the characteristics of

those systems which affect integration.

To be more specific, certain types of materials, for example, are more suited to a

given range of pressures, temperatures, and chemical and physical compositions than

others. Matching the ranges of these parameters in the plant design envelope to

suitable materials is usually thought of as the process plant designer’s job. Similarly, the


selection of pumps, heat exchangers, instrumentation, valves, and so on is usually

thought to be part of process plant design.

The information required to make these selections is largely absent from chemical

engineering degrees, justified by the idea that it is mere qualitative data which is

insufficiently intellectually demanding for university level education.

Similarly, the qualitative criteria used to choose between separation processes and

other technologies are frequently thought of as too shallow and easy to be worth a

student’s time. Students mostly concentrate on a few selected processes which can be

used to illustrate scientific principles or mathematical techniques.

Practitioners understand that such knowledge is actually capable of forming

the basis of quite subtle and sophisticated multidimensional reasoning, and that

providing such judgments is one of the basic expectations of a process plant design

engineer.

I will attempt in the following chapter to provide matrices showing a number

of dimensions which may be used to choose between options for materials of

construction, valves, pumps, blowers compressors and fans, separation processes, and

heat exchangers. I will also offer information on specification of electrical components

and instrumentation.


At conceptual design stage it is often important to know at a category level what

kinds of components we are thinking of using. Whether we are going to use

rotodynamic or positive displacement pumps, membranes or distillation, globe or

butterfly valves, carbon steel or plastic is usually known to the process plant

designer by the time their initial drawings are done. All of these decisions affect the

fundamental characteristics of the design and have implications for cost, safety, and

robustness.

Experienced professionals might not even know they are making some of these

choices (which might be strongly affected by custom in the sector and personal prefer-

ence), but the beginner has to make conscious choices. There may be little formal

documentation of some of these choices at this early stage, but the designer has to

make many of them to carry out even a conceptual design.

At the detailed design stage we are selecting specific commercially available items

of equipment. We produce datasheets which set out the detailed specification of

the item, or incorporate our choices in the case of materials of construction.

Manufacturers may, on sight of these datasheets, feed back to us information which

allows us to refine or reconsider our choices.


More generally, the more detailed analysis may show some of our conceptual

design choices to be less than ideal, so we might change our minds. If someone else

did the conceptual design, it can be a good idea to ask them why they chose the

component which you want to change. They may have had a good reason to choose

it based on factors you are unaware of. If that is not possible, the project design

philosophies may be of assistance.

Design for construction generates a lot more detailed documentation, and experi-

enced engineers will be likely to review your design choices before this is finalized.

They may ask for changes based on their experience of what works, or more impor-

tantly still, what does not.

MATERIALS OF CONSTRUCTION

Plant designers need to know which materials are going to be suitable for the duty to

which they intend to put the plant, as well as the duties to which it might (inten-

tionally or unintentionally) be put.

This is not determined so much by material science as by practical experience, and

a broad qualitative knowledge of available materials and their strengths and limitations.

There are also traditional default positions in various process sectors. For example,

the generic pipe material is carbon steel in the oil and gas industry, and plastics in the

water industry—even highly corrosive water is transported in carbon steel piping in

the oil and gas industry.

Though this seems odd to a water specialist, this is not wrong, as long as suitable

corrosion allowances are made, and the consequent increased metal ion content of the

water is acceptable from a process point of view (Table 10.1).

131How to Design and Select Plant Components and Materials

Table 10.1 Materials of constructionRelativeprice

Temperaturerating

Pressurerating

Waterresistancea

Organicsolventresistance

Acidresistance

Alkaliresistance

Abrasionresistance

Chlorideresistance

UVresistance

Hardness Toughness

Metals

Cast iron L H M/H M H L L/M H L VH H M/H

Carbon steels L H H L H L L M/H L VH H H

Stainless

steels

M H VH H H L L/Mb VH L/Mb VH H VH

Bronzes M M/H H L H L L M M VH M M

Brasses M H H H H L VL M M VH M M

Hastelloy VH VH VH VH H H M H H VH H H

Tantalum VH VH M/H VH H VH M H H VH H VH

Inconel VH VH VVH VH H M/Hb H H M/Hb VH H H

Aluminum M/H M/H M/H H H L L M L VH H H

Titanium H H M/H H H M L H VH VH H H

Precious

metals

VVH H L/M H H M/H VH L/M VH VH L/M L/M

Plastics

PVC L VL/Lb M/Hb H Mb H H M H VL M H

ABS L VL L H M H H M H M M H

PP/PE L VVL VL H L H H L/M H H L M

PS L VL L/M H L H H L H H L H

Acrylic M L L/M H H H H H H H

Nylon M L/M M M/H L/M M H H H H M H

Polybenzi-

midazole

H M/H M H H L/M L H H H M H

Fluoropolymers

PTFE H M VL H H H H M H H M/H M

PVDF VH M L/M H H H H M H H M/H M/H

Other

Ceramics M VVH VVH H H L/M L VH H VH VH L

Graphite H H L/M H H H L/M L H H L L

Glasses M M L/M H VH H M H H H VH L

Rubber L L L/M H L/M L H H H H L H

Composites H L H H M/H L/M H M H M M H

aCorrosive water by Langelier index (LSI).bVaries by grade.

Scaling and corrosionPrediction of the scaling or corrosive nature of fluids impacts on many areas of design

from the selection of tube or shell side duty on a heat exchanger of a fluid to fouling

factors and corrosion allowances. There is a lot to this subject, but I will confine

myself to covering a few key aspects in outline.

Corrosion allowances are temperature, pressure, and internal and external chemical

environment dependent, and limited life stipulations may be needed for pressure ves-

sels in corrosive environments.

Effects such as erosion by entrained solids, galvanic corrosion, stress cor-

rosion cracking, hydrogen embrittlement, cavitation, vibration, thermal expansion/

contraction, and water hammer may all cause the premature failure of metal

components.

New processes especially might need extensive testing of materials. Materials

specialists should assess suitability of materials of construction at detailed design

stage. For earlier stages of design, Table 10.2 (from “Practical Process Engineering”)

may be useful.

There are quite a number of indices used to predict whether water is going to be

scaling or corrosive, which are all at best quite rough heuristics.


Table 10.2 Corrosion table

Brass

Bronz

e

Allo

y20

Hastello

yC

Mon

el

304stainlessstee

l

316stainlessstee

l

Titanium

Silicon

iron

Tantalum

Cop

per

Aluminum

Carbon

stee

l

Butyl

rubber

Epox

y

Hyp

alon

Natural

rubber

Neo

prene

Nitrile

rubber

Nylon

Phen

olic

Polyethy

lene

Polypropylen

e

PVC

Silicon

eelastomer

TFE

Ceram

ic,a

lumina

Graphite

Acetaldehyde A A A A A A N N N A A N B N A A A

Acetic acid, 20% C C A A B A A A A A A A N A A A C C N C A A A A A A

Acetic acid, 80% C C A A B A A A A A A A N A B A C A N C A C A A A

Acetic acid, glacial C C A A B A A A A A A N A B N C C A N A C B N A A A

Acetic anhydride C C B A A B A A A A C A A N B A A N A A A

Acetone A A A A A A A A A A A A A C C C N C N A A A C N N A A

Aluminum chloride C C C A N N C C C A C C N A A A A N N A A A A C A A A

Aluminum sulfate C C C A C C C A A A C C N A A C A C A A A A A A A

Ammonia, 10% N N A A N A A A N C C C C A N A N A A A A A A

Ammonium chloride N N C A C C C A C A N C C A A N A N N A A A A A A A

Ammonium nitrate N N A C N A A A A A N A C A A A A N A A A A A A A

Ammonium phosphate N N A A N A A A A C C C C A A A C A A A A A A A

Ammonium sulfate N C A A C A C A A A C A C A A C N A N N A A A A A A A A

Amyl acetate A A A A A A B N N N C N N N A A A

Amyl alcohol B A A A A A A A A A C B A A A A C A A A

Amyl chloride B A A A A B A A N C N B C N N N A A A

Aniline N N A A A A A A C A N N A B B N N B N B N A A A

Aqua regia N N N C N N N A N A N N N N C N N B N N A A

Arsenic acid N B B B B B A C N A A N B A A A A A

Barium chloride A A A A A B B A A A A N A A B C A A A A A A A A A A

Barium sulfate A A A A A A A A A A A A A A A C A N A A A A A A A

Beer A A A A A A A C A A A N A A A A A A

Benzaldehyde A A A A A A A A A A A A N N N N A B A B N A A A

Benzene A A A A A A A A A A A N B N N N N A A N B N N A A A

Benzoic acid C A A C A B A A A N A N N C A C A A N C A A

Borax A A A A C A A A A A A N A A A A A A A A A A A

Boric acid N A A A A A A A A A C A N A A A A A N A A A A A N A A A

Bromine water C C N A C N N A N C N N B N N N N N B A A N

Butyl acetate A C A A C C A A A A B B N N A N N N N A A A

(Continued)

Brass

Bronz

e

Allo

y20

Hastello

yC

Mon

el

304stainlessstee

l

316stainlessstee

l

Titanium

Silicon

iron

Tantalum

Cop

per

Aluminum

Carbon

stee

l

Butyl

rubber

Epox

y

Hyp

alon

Natural

rubber

Neo

prene

Nitrile

rubber

Nylon

Phen

olic

Polyethy

lene

Polypropylen

e

PVC

Silicon

eelastomer

TFE

Ceram

ic,a

lumina

Graphite

Butyric acid B A A A A C A B A A A B C N A N N N A C A A A

Calcium bisulfate N A C A N C C C N A A A A A A N A A A A A A N A A A

Calcium chloride A A A C A A A C A A A C A A N A A A A N A A A

Calcium hypochlorite C A C C C C C A A A C C C B A B N N A A A A A A

Furfural A C A A A C C A A A A A A B B N A A N N N A A A

Gasoline A A A A A A A N A A A A C N C N N N A A A N N B C A A A

Glycerine A A A A A A A A A A A A A A A C A A N A A A A A A A A A

Heptane A A A A A A A A A A A A A C C B C A N B A A A A

Hexane A A A A A A A B C B A A N B B A A A

Hydrobromic acid, 20% N N N A N N N A N A N N N C A N C N A A A A B A

Hydrochloric acid, 0�25% N N N C C N N C A A C N N C A A N A C N A A A A C A B A

Hydrochloric acid, 25�37% N N N C C N N C A A C N N C A A N A C N A A A A N A B A

Hydrocyanic acid N N A A A A A A A N N N A A A A A A B A

Hydrofluoric acid, 10% N N B A A B B N N N N N N A A A A A N N A A B A A N A

Hydrofluoric acid, 30% N N B A A B B N N N N N N A A A A A N N A A B C A N A

Hydrofluoric acid, 60% N N B A A B B N N N N N N A A C A N N N A A A C N A N A

Hydrogen peroxide, 30% N N A A C C A A C A C C C B A N C N A B A A A A

Hydrogen peroxide, 50% N N A A C C A A C A C C C B A N C N N B B C A A

Hydrogen peroxide, 90% N N A A C A A C A C C C N C N N A N A B B N C A A

Hydrogen sulfide, aqueous N N A A N C A A A A N A C A A A N A A A A A B A A A

Iodine in alcohol N A A A N N N N N N N N B N C A A

Kerosene A A A A A A A A A A A A A N N N A N C A A A A

Lactic acid A A A A B B A A A A A N A A A A C A A A A A A

Lead acetate A A A A A A A A A N A N A A A A A A A

Magnesium chloride C C A A A A A A A C C C A A A A A N A A A A A C A A

Magnesium nitrate A A A A A A A A C A A A A A A A

Magnesium sulfate A A A A A A A A A C C A A A A N A A A A A C A A

Maleic acid A A A A A A C A A A A A A A A A

Methanol A A A A A A A A A A A A A A A A A B B A B A A A

Methyl chloride C A A A A A A A A N N N N N C C N N N N A A A

(Continued)

Table 10.2 (Continued)

Brass

Bronz

e

Allo

y20

Hastello

yC

Mon

el

304stainlessstee

l

316stainlessstee

l

Titanium

Silicon

iron

Tantalum

Cop

per

Aluminum

Carbon

stee

l

Butyl

rubber

Epox

y

Hyp

alon

Natural

rubber

Neo

prene

Nitrile

rubber

Nylon

Phen

olic

Polyethy

lene

Polypropylen

e

PVC

Silicon

eelastomer

TFE

Ceram

ic,a

lumina

Graphite

Methyl ethyl ketone A A A A A A A A A A A A A A B N N A N A B N A A A

Methylene chloride N N A A N A A A A A N N N N N N N A A A

Napthalene A A A A A A A A A A A N N N A A B N A A A

Nickel chloride N N A A A A A A A A N A A A A A N A A A A A C A A A

Nickel sulfate C A A A A A A A N A A A A A N A A A A A C A A A

Nitric acid, 10% N N A A N A A A A A N B N A A B N N N N A A A A A

Nitric acid, 20% N N A A N A A A A A N B N A A B N N N N A A A A A

Nitric acid, 50% N N A A N C C A A A N B N N N N N N N C N B A N A A N

Nitric acid, anhydrous N A A N C C A A A N B N N N N N N N A N N N A A A

Oleic acid A A A A A A A A A A C A C C C N A B A A A A

Oxalic acid A A C A A C B A A A C C C A A A N A C A A A A

Phenol A C A A C C A A A N B N C N B N A A B A A A

Phosphoric acid, 0�50% C C A A C C C B A A C N C A A C A N N A A A A A A A

Phosphoric acid, 50�100% C C A A C C C B A C N C C A C A N N A A A C N A A A

Potassium bicarbonate A A A A A A A A A A A A A A A A A

Potassium bromide A A A A A A A A A N A A B A C A A A A A A

Potassium carbonate A A A A A A A A A N A N A A A A A C A A B A A C A A A

Potassium chlorate N A A A A A A A A A A A C A N C A A A A C A A

Potassium chloride C C C A A C C A A A A A A A A A A N C A A A A C A A A

Potassium cyanide N N A A A A A A A A N N A A A A A A A A A A A

Potassium dichromate N N A A A A A A A N A A A A A N A A A A A

Potassium hydroxide N N A A A C C C N N N N C A A C C A C A A A A A A A N A

Potassium nitrate N A A A A A A A A A A A A A A A A N C A A A A A A A A

Potassium permanganate A A C A A C C A A A A A A A A N A A A A A A

Potassium sulfate C C A A A A A A A A A A C A A A A A N C A A A A A A A

Propyl alcohol A A A A A A A A A A A A B B A A

Sodium acetate A A A A A A A A A A A A A A A A A A A A A

Sodium bicarbonate A A A A A A A A A A A A N A A C A N A A A A A N A A A

Sodium bisulfate N C A A A A A A A N C C N A A C A N C A A A A C A A A

Sodium bisulfite A A C A A C C A N A A A A C A A C A A A A A A

(Continued)

Brass

Bronz

e

Allo

y20

Hastello

yC

Mon

el

304stainlessstee

l

316stainlessstee

l

Titanium

Silicon

iron

Tantalum

Cop

per

Aluminum

Carbon

stee

l

Butyl

rubber

Epox

y

Hyp

alon

Natural

rubber

Neo

prene

Nitrile

rubber

Nylon

Phen

olic

Polyethy

lene

Polypropylen

e

PVC

Silicon

eelastomer

TFE

Ceram

ic,a

lumina

Graphite

Sodium carbonate A A A A A A A A A N A N A N A A A A N A A A A A N A A A

Sodium chlorate A C C C C A A A A C C A N C A A A A C A A

Sodium chloride C C C C A A C A A A A A A A A A A A A

Sodium cyanide N N A A A A A A N N A A A A N A A A A A C A A A

Sodium hydroxide, 20% N C A A A A A A N N C N A A A A C A A A C A A A A A N A

Sodium hydroxide, 50% N C A A A A A A N N C N A A A C C A A A C A A A A A N A

Sodium hypochlorite N C C C C C C A N A C N N A A C C N N A A A A C A A A

Sodium nitrate C C A A A A A A A A A A A A A A C A N A A A A C A A A

Sodium silicate A A A A A A A A A A A A A A A A A A A A A A A A A

Sodium sulfate A A A A A A A A C A A A C A A A C A N A A A A A C A A A

Sodium sulfide N N A A C C C A A N C N C A A A C A N A A A A A C A A A

Stannic chloride N B C C N N A N A N N A C C A N A A A N A A A

Stearic acid A A A A B A A A A A A A A A C N C A B A A A A

Sulfuric acid, 0�10% N A A A C N N B A N N N N A A N A N N A A A A A A A

Sulfuric acid, 10�75% N A A A C N N C N A N N N N B A N N N N C C A A N A A A

Sulfuric acid, 75�100% N A A C C N N N N A N N N N N C N N N N C C B B N A A A

Tannic acid A A A A A C C A A A A N A A A A N A A A A A A

Tartaric acid C C A A C A A A A A A A A A A A A A A A A A

Tetrahydrofurane A A A A A N B N A A

Toluene A N A A A A A A A A A A A N C N C N N A A N B N N A A A

Trichloroethylene, dry A N A A A A A A A A A A A N A N N N N N C N B N N A A A

Turpentine C C A A A A A A A A C A C N B N N N C A A N B A N A A A

Urea A A A A A A A A A A A A A A A

Xylene A A A A A C N N A A N N N A A A

Zinc chloride N N A A A A A A A A N N N A A A C A N N A A A A A A A A

Zinc sulfate C A A A A A A A A A C C A A A C A N C A A A A A A A A

A, acceptable; B, acceptable up to 80�F; C, caution, use under limited conditions; N, not recommended; blank space, effect unknown.

(Reproduced from Sandler, H.J. and Luckiewicz, E.T. (1987) Practical Process Engineering: AWorking Approach to Plant Design).

I usually favor the Langelier index (LSI) for historical reasons but for carbonate buffered

systems the Ryznar/Carrier Stability Index (RSI) is supposedly more empirically based.

The Larson-Skold index predicts corrosion of mild steel and, since it considers

sulfate and chloride as well as bicarbonate, is commonly used to predict corrosivity of

once-through cooling seawater.

The Oddo-Tomson index allows for water, gas, and oil phases, and the effect of

pressure on CO2 saturation, to satisfy oil and gas industry needs.

MECHANICAL EQUIPMENT

Though the average new graduate knows very little about pipe specification and so

on, I will focus in the section on qualitative information on the most important items

of mechanical equipment which are largely untaught in chemical engineering degrees.

These are in my opinion fluid transport, flow control and heat exchanger selection.

Pumps/blowers/compressors/fansFluid moving equipment generally comes in two main varieties—rotodynamic or

positive displacement. Each of these comes in many subtypes, but beginners have

often not been taught the crucial differences between the two broad types.

Later, I provide a table giving you some ideas on how to choose between the

commonest types of liquid and gas moving equipment, but first let’s see how we

choose between the broad varieties of pumps (Table 10.3).

This is why we tend to use positive displacement pumps for metering duties, and

centrifugal (rotodynamic) pumps for moving large volumes of flow at relatively low

pressures. More detailed choices can be given in Table 10.4.

Table 10.3 Pump selection (general)Rotodynamic Positive displacement

Head Low—up to a few bar High—hundreds of barSolids tolerance Low without efficiency losses Very high for most types

Viscosity Low viscosity fluids only Low and high viscosity fluidsSealing arrangements Rotating shaft seal required No rotating shaft seal

Volumetric capacity High LowerTurndown Limited Excellent

Precision Low—discharge proportionalto backpressure

Excellent—dischargelargely independent

of backpressure

Pulsation Smooth output Pulsating output

Resistance to reverse flow Very low Very highReaction to closed valve

downstream

No damage to pump Pump damage likely


Table 10.4 Pump selection (detailed)Relativeprice

Environmental/safety/operabilityconcerns

Robustness Shear Maximumdifferentialpressure

Capacityrange

Solids handlingcapacity

Efficiencya Sealin-out

Fluids handledb Self-priming?

Rotodynamic

Radial flow M Cavitation H H M/H L-VH M H M Low viscosity/

aggressiveness

N

Mixed flow M Cavitation H H M M-VH M H M Low viscosity/

aggressiveness

N

Axial flow M Cavitation H H M M-VH M H M Low viscosity/

aggressiveness

N

Archimedean

screw

H Release of

dissolved gases

VH H L M-VH H L N/A Low viscosity/

aggressiveness

N

Positive displacement

Diaphragm M Overpressure

on blockage

VH L M/H VL-M H L H Low/high viscosity/

aggressiveness

Y

Piston diaphragm H Overpressure

on blockage

M L VH VL-M L/M M H Low/high viscosity/

aggressiveness

Y

Ram H Overpressure

on blockage

H M VH M-H H M H Low/high viscosity/

aggressiveness

Y

Progressing cavity M Overpressure

on blockage

M VL H M-H H H H Low/high viscosity/

aggressiveness

Y

Peristaltic H Overpressure

on blockage

L VL M VL-M H M/H M Low/high viscosity/

aggressiveness

N

Gear L Overpressure

on blockage

L M VH VL-L L L H Low/high viscosity.

Low aggressiveness

Y

Screw L Overpressure

on blockage

L M VH L-M L L H Low/high viscosity.

Low aggressiveness

Y

Other

Air lift L VH VL VL L-M H L L Low viscosity, low/

high aggressiveness

N

Eductor M Blockage of

eductor

M M VL L-H M H M Low viscosity, low/

highaggressiveness

Y

aCentrifugal pump efficiency reduces as viscosity increases, but PD pump efficiency increases. Centrifugal pump efficiency is more to do with impeller type than anything else, impeller type is determined byprocess conditions such as any solids handling requirement.bAggressiveness is related to presence of abrasive particles, undissolved gases, or unfavorable LSI.

ValvesWe can think of valves in a number of broad categories. It can incidentally be helpful,

when first constructing a Piping and Instrumentation Diagram (P&ID) of a process,

to add them using these categories in turn.

The most common types on a process plant are usually the valves which allow

every item of equipment to be capable of isolation from the rest of the plant for main-

tenance purposes. The (usually manually operated) valves we use to do this can be

called isolation valves. They are usually set to either their fully open or fully closed

position, rather than being used for flowrate control. These are usually tagged on a

P&ID as MV—manual valve (though some think nowadays that this clutters the

P&ID, and do not tag MVs).

A variant on isolation which can be useful (but carries a significant risk) is

a bypass. A bypass valve allows a unit operation to be bypassed. If a complete or

partial bypassing of a unit operation carries safety or performance implications (as it

usually does), the designer needs to think about how to protect the plant from

accidental or deliberate bypassing of a unit operation by operators. Valves can be

locked out, or a section of pipe known as a spool piece can be left out and kept

under lock and key to make sure that bypassing is only done deliberately and under

management control.

Bypassing of actuated control valves via a manual control valve is more akin to a

manual standby unit, and is far less risky from a process point of view than bypassing

of unit operations.

There are a smaller number of valves on a plant which are used to control flow

(control valves). They may be used for on/off control, or they may be used to modu-

late flow by intermediate degrees of opening. The setting of these valves may be man-

ual, or more frequently nowadays they are moved by means of motors known as

actuators, controlled by computer. These may be tagged on a P&ID as AV—actuated

valve, or less desirably FCV—flow control valve. They should not be tagged as MV—

motorized valve, to avoid confusion with the last category.

Then there are the self-operating safety valves, including nonreturn valves, pressure

relief valves, and pressure sustaining valves.

My final category is externally operated safety valves such as emergency shutdown

valves.

The preceding list is of valve duties rather than valve types. In order to choose an

appropriate type of valve for a duty we need to know the characteristics of the various

types of valves commercially available. Table 10.5 gives this information to allow a

choice to be made.

There is also a handy table in Practical Process Engineering (see Further Reading)

which I reproduce in Table 10.6.


Table 10.5 Valve selection

Relative

price

Robustness

Sizes(m

mNB)

Materialsof

construc

tion

Fluidsha

ndled

Solid

sha

ndlin

gab

ility

Seal

in-out

Seal

up-dow

nstrea

m

Con

trollability

Actua

tortype

Pressure

rating

a

Man

ufacturerb

Isolating

Butterfly/disc L M 50�500 Plastics, cast iron M M M M/H 1/4 turn cylinder,

electric motor

M/H Bray

Globe L M 50�600 Brass M L/M M M/H Multiturn electric

motor, linear

cylinder

M Vela

Ball L H 5�1,200 Plastics M/H M H M/H H George

Fischer

Diaphragm H H 15�500 Plastics, stainless steel,

aluminum

H H H L/M M Saunders

Gate M M 50�1,250 Cast iron H L/M H M/H H GWC

Control

Needle H M 6�25 Stainless steel L M H H H Hoke

Globe L M 50�600 Brass M L/M M M/H M Velan

Plug H M 15�1,800 Cast iron, ductile iron, bronze,

aluminum, carbon steel,

stainless steel, alloy 20,

and Monel

Clean and dirty liquids and

gases, sludge, and slurries

H M M/H H Lever, hand wheel,

chain wheel,

cylinder, electric

motor

M De Zurik

V-Notch ball L H 6�150 Plastics, stainless steel M M H M/H M/H George

Fischer

Eccentric disc H M 50�1,200 M M H M/H M Neles

Other

Emergency

shutdown

H H 50�400 M M H H M/H Becker

Pressure relief H M 6�900 Brass, cast iron, stainless steel L L/M L/M NA M Farris

Swing check M H 50�900 Cast iron H M L/M NA M Velan

Spring check M M 6�600 Cast iron M H M NA M Flowserve

Pressure

sustaining

H M 6�350 Plastics L M/H � M/H M Bermad

3-Way ball M M 12.5�150 Plastic, cast iron M/H M H M/H M George

Fischer

3-Way plug M/H H 80�400 Cast iron, Ni-resist, aluminum,

carbon steel, 316

stainless steel

Clean and dirty, viscous and

corrosive liquids, sludge,

abrasive and fibrous

slurries, and clean and

dirty corrosive gases

H M M/H M/H Lever, hand wheel,

chain wheel,

cylinder,

electric motor

M/H DeZurik

aThere are ISO/API classes of valve by pressure/temperature ratings.bNot a recommendation, just a route to more information.

Table 10.6 Primary usages for various common valve typesLiquid flows Gaseous flows Solid flows

Neutral(water,oil, etc.)

Corrosive(alkaline,acid, etc.)

Hygienic(beverages,foods, drugs)

Slurry Fibroussuspensions

Neutral(air, steam,N2, etc.)

Corrosive(acid vapors,chloride, etc.)

Vacuum Abrasivepowder

(silica, etc.)

Lubricatingpowder(graphite,talc, etc.)

Type On-off Reg On-off Reg On-off Reg On-off Reg On-offand Reg

On-off Reg On-off Reg On-off Reg On-offand Reg

On-offand Reg

Gate • • • • • • • •Globe • • • • • • •Ball • • • • • •Plug • • • • •Butterfly • • • • • • • • • • • • •Diaphragm • • • • • • • • • • • • •Flush-bottom • • •Squeeze • • • • • •Pinch • • • • • •Needle • • •Slide gate •Spiral sock • •

(Reproduced from Sandler, H.J. and Luckiewicz, E.T. (1987) Practical Process Engineering: AWorking Approach to Plant Design).

Heat exchangersAll new graduate chemical engineers can perform a “shortcut design” for a heat

exchanger, but few really know of more than one type—the shell and tube exchanger

which the shortcut method applies to.

The kind of detailed design taught in universities is usually a matter for equipment

suppliers rather than process plant designers. There are, however, quite a number of

types of heat exchangers, and selection between them is part of the process plant

designer’s job.

Table 10.7 is intended to help the beginner to select a suitable heat exchanger

type or types.


Table 10.7 Heat exchanger selectionRe

lative

initialcost

Robustness

Sizes(kW)

Materialsof

construc

tion

Fluidsha

ndled

Solid

sha

ndlin

gab

ility

Foulingresistan

ce

Maintaina

bility

(clean

ing)

Sealing

Max

design

pressure

Max

design

temperature

Hyg

ienicop

eration?

Effic

ienc

y

Footprint

Requiredtemperature

ofap

proach

Pressure

drop

Shell and

tube

M VH H Flexible,

depends on

corrosion

study

Liquid, gas

or two-

phase

M; To select

appropriate

pitch and type

L M; To select

appropriate

pitch and type

depending

on service

H H H N M H H L

Spiral VH Y

Double pipe L (for

low

duty)

VH L Flexible,

depends on

corrosion

study

Liquid, gas

or two-

phase

H L H H H H Y L VH H L

Printed

circuit

heat

exchanger

H VH H Flexible, depends

on

corrosion

study

Liquid, gas

or two-

phase

L; Depends on

particle size

and

flow passage

size; Prior

filtration may

be required.

Generally only

for clean

service

H VL (chemical

cleaning only;

mechanical

cleaning is

not possible)

VH VH VH N VH VL VL VH

Plate and

shell

VL H (plate

failure)

H Ensure fluid

handled

compatible

with gasket

Normally

Liquid,

rarely gas

or two-

phase

L; Prone to

blockage

H VH M M H Y H L VL (as low

as

1�C)

H

Plate and

frame

VL L (plate

failure,

gasket

failure)

H Ensure fluid

handled

compatible

with gasket

Normally

liquid,

rarely gas

or two-

phase

L; Prone to

blockage

H VH L L (up to

25 bar g)

L (Design

temperature

limited by

gasket

material)

Y H L VL (as low

as

1�C)

H

ELECTRICAL AND CONTROL EQUIPMENT

Chemical engineering students do not really think about power and control for

the systems they “design,” other than in the most abstract mathematical terms. An

understanding of the needs of electrical and control engineers is, however, crucial to

competent design.

Here are the most important items for consideration:

Motor control centersWhen I first started designing plants, I did not know what a Motor Control Center

(MCC) was, why it was needed, and what it contained. (This is a standard feature of

Chemical Engineering degrees: we don’t get taught about the most basic needs of the

other disciplines we will be interacting with.) This became a bit of a problem when I

almost won a job for my employer in which I hadn’t thought it necessary to include

an MCC. To save you the same embarrassment, allow me to explain.

Electrical motors or “drives” require maybe six times their running power to start

them up. Rather than uprating all the power cabling and so on to this starting current,

motor starters are used to send a pulse of power to get the drive spinning. They also

contain overload protection and so on.

Direct on Line (DOL) starters are very cheap, but they simply apply the full line

current to the motor all at once in a way which usually limits their use to drives rated

at less than 11 kW.

Star Delta starters are more expensive. They apply current to the motor in two

configurations in a way that reduces starting torque by a factor of three. These are

probably required above an 11 kW drive rating.

Soft Starters are the most expensive type. They control voltage during drive start-

up in a way which avoids the torque and current peaks associated with DOL and Star

Delta starters, and have some of the sophisticated control functions of the variable

speed drive (VSD).

Inverters or VSDs are perhaps a little more expensive than Star Delta starters, but

they have a lot more flexibility. They allow very sophisticated patterns of power ramp-

ing to be applied to the drive on start-up, as well as allowing variable frequency to be

supplied in a way which allows drive rotational speed to be controlled. They always

have a microprocessor on-board nowadays so that multiple, quite sophisticated control

loops and interlocks can be run directly through them.


These drives are usually collected in a big box called an MCC, which will usually

contain internal subdivisions housing starters or groups of starters. There are some

other common features, as shown in Figure 10.1:

The image shows a “Form 4” panel, with an intermediate degree of separation between

controls for different aspects of a process. There are four forms of panel specified in IEC

60439-1: 1999, Annex D and BS EN 60439-1: 1999. There are lettered subtypes, but

broadly:

Form 1 has no separation, and is often referred to as a wardrobe type. Failure of

one component in a Form 1 panel can damage other components, and a single

failure will take the whole process offline.

Form 2 separates the bus bars (big copper conductors which carry common main

power throughout the panel) from other components. Not much better than Form 1.

Form 3 separates the bus bars from other components, and all components from

each other. This is the minimum specification if you would like sections of the

plant to be capable of running while one of them is offline.

Form 4 is as Form 3, but also separates terminals for external conductors from

each other.

Figure 10.1 Photograph of an MCC unit, showing incomer, starters, marshalling cubicle, and PLC/UPSsections. Copyright image reproduced courtesy of the Process Engineering Group, SLR Consulting Ltd.


There is a different system in the United States (described in standard UL 508A)

which takes a different approach, but addresses the same issues. In either case the

process designer will be required to specify broadly which kind of panel they want,

though clients may specify a minimum separation between control switchgear.

Panels will also have a specified degree of ingress protection (IP55 is usually the

minimum standard).

Consideration will also need to be given to direction of cable entry, which can be

from the top, bottom, or a combination of both. Bottom entry requires the panel to

sit on a channel in the floor, so there are civil engineering implications.

There is great deal more to this, but this is the minimum level of knowledge

required of a process designer to integrate their MCC design.

CablingAnother thing which generally receives no attention in chemical engineering degrees

is cabling. The absolute minimum information you need to know about cables is as

follows:

Every instrument needs incoming power and outgoing signal cabling. Every

electrical drive (motor) on the plant needs incoming power cabling. Every MCC

needs incoming power cabling and outgoing power and signals cabling.

Power cable size is calculated by electrical engineers in a similar way to pipe size.

The more current a cable carries, the thicker it has to be to avoid overheating. A

complicating factor is that cables can have a variable number of wires or “cores”

inside them. Thus we can have a thick cable with a single set of large cores feeding a

single large drive, or a similarly thick cable with multiple smaller cores to feed a num-

ber of drives.

Instrument cabling needs to be arranged so as to be unaffected by electromagnetic

fields from the power cabling. This is usually achieved by some combination of physi-

cal separation and shielding. 1 m of separation will usually do it, but your electrical

engineer will advise.

The basic kind of power cable is the relatively inflexible SWA (Steel Wire Armored)

PVC-insulated type. There is also a more flexible, unarmored, and waterproof kind used

to connect submersible pumps. Instrument cabling also comes in a number of types, and

is a lot smaller than power cabling as it is only carrying 5�240 V, and no real power.

Cables have a minimum bend radius. The thicker the cable, the bigger this is.

There needs to be space in the design to accommodate this. It is approximately equal

to the radius of a commercially available long radius bend in a pipe large enough to

carry the cable.

All kinds of cable are carried underground through ducts, which are nowadays

plastic pipes. These may be cast into concrete slabs, unlike pipes carrying fluids.


We usually specify a few more such ducts than we need in a design to allow for future

expansion. Cables are carried overground on (usually elevated) cable trays. Power and

signals cabling should ideally be in separate ducts and cable trays.

InstrumentationThere are many specialized process instruments, but the four commonly measured

parameters are pressure, flow, temperature, and level, and process designers need to be

able to choose between the most common types of instrumentation used for measur-

ing these things. Table 10.8 should help:

Table 10.8 Instrumentation selectionRelative price Robustness Contact with

process fluid?Solids handlingability

Pressure instruments

Bourdon L L Y M

Capacitance M H Y H

Resistance L M Y HPiezoelectric M H N H

Optical H M N M

Flow instruments

Variable area L L Y L/M

Mechanical L L YPressure M M Y L/M

Electronic M/H H N HRadiation H H N H

Vortex H H N H

Doppler M H N VH

Temperature instruments

Thermocouple L H Y H

Bimetal L M Y M/H

Resistance L H Y H

Level

Sonar/Radar M M N H

Float L M Y HPressure M M Y M/H


Control systemA number of control systems are used in process plant design. Selection between them

for whole-system or local design may be as much a matter of client preference, indus-

try familiarity, and designer preference as inherent characteristics of the system type.

Local controllersOnce upon a time, control loops were operated by mechanical, electromechanical,

pneumatic, or electrically operated boxes which were mounted locally to the thing

being controlled.

By the time I was at university, these were most commonly solid state electronic

PID (Proportional, Integral, Differential) controllers (Figure 10.2).

We still use the odd dedicated field-mounted controller (for pH control, for

example) but process plant designers never do the thing we were taught to do in

university—writing algorithms for these boxes. They are products, whose manufac-

turers have done this job for us. Their limited configurability also makes these con-

trollers the most robust solution. Writing software is best left to experts.

Figure 10.2 Wall-mounted PID controller. Copyright image reproduced courtesy of Amot.


The oil and gas industry still use the modern equivalent of P&ID controllers, and

a DCS system to facilitate their aftermarket optimization and tuning of plants, but this

is not much to do with process plant design as I define it here.

Programmable logic controllersIn my industry, programmable logic controllers (PLCs) are commonly used for whole

system control. PLCs are a kind of computer which is custom built out of com-

ponents to suit a particular duty.

There is a range of central processing units of increasing power available, and rack-

mounted cards are added to this to provide a suitable number of input and output

channels.

Direct interface with a PLC is via a human�machine interface (HMI) or PC

program. These can vary in appearance from the program’s green-screen and ladder

logic to sophisticated simulation interfaces of supervisory control and data acquisition

(SCADA) systems.

It should be noted that, while PLCs themselves cannot be directly infected with

computer malware, the intelligence community produced a worm (Stuxnet) which

can attack PLCs via their SCADA connected PCs (thought to have been written to

target the Iranian nuclear program). Stuxnet was purely destructive, but a more recent

virus called Duqu is a keystroke-logging spyware program.

Any systems which include a PC may be compromised, especially as PCs are

almost always connected to the internet nowadays, and site communications and

signals are increasingly connected via Wi-Fi rather than hard wired.

PCSupervisory control and data acquisitionSCADA systems run on a PC. They can receive signals from one or more PLCs, or

from remote telemetry outstations (RTUs) which convert 4�20 mA signals from field

instruments into digital data.

Such signals may be carried by local or wide area networks using internet

protocols, by telephone lines or satellite signals.

The SCADA system has a HMI—usually in the form of simulation screens which

look rather like animated PFDs, alarm handling screens, “trends” screens which allow

variation in parameters to be seen as a graph against time, and input screens which

allow process parameters to be changed (by authorized users).

DCSDCS used to be more different from SCADA than it is nowadays. Historically

SCADA used dumb field instrumentation, and had a centralized system brain, whereas

DCS has a lot more control out in the field.


As it is getting increasingly difficult to buy dumb instrumentation, or even dumb

motor starters, the distinction is not as sharp as it was, but DCS is more likely to

involve field-mounted controllers. These are less likely to be simple PID controllers,

but will be capable of more sophisticated control.

The oil and gas industry makes extensive use of DCS, which is well suited to their

culture of radical postpurchase control system tuning.

Aftermarket systems: supervisory computer, etc.In the oil and gas industry, there is a tendency to install postpurchase a supervisory

control system running advanced control software, such as MPC (multivariable pre-

dictive control) and RTO (real-time optimization). To quote Myke King:

MPC is installed on pretty much every oil refinery and petrochemical site, but it’s not some-thing that needs a lot of attention at the process design stage.

It and RTO (if justified) would typically be engineered well after process commissioning—in some cases 30 years after commissioning! There would be other PCs connected to the DCSfor process data collection—typically based on a real-time database (such as OSI’s PI,Honeywell’s PHD etc.). They would have links to other process management systems—such asLIMS (laboratory information management system).

FURTHER READINGBranan, C., 2012. Rules of Thumb for Chemical Engineers. Elsevier.Couper, J.R., Penney, W., 2012. Chemical Process Equipment—Selection and Design. Elsevier.




CHAPTER 11

How to Design Unit OperationsINTRODUCTION

Process plant designers very rarely carry out detailed design of unit operations, as

they do not intend to offer the process guarantees for the units. Detailed design is

a job for those guaranteeing the performance of the unit operation.

The plant designer’s job is to get their design correct enough such that the unit

operation will work reliably across the design envelope, as the process guarantee will

limit its validity to that design envelope. We also frequently check that there are no

misunderstandings in what vendors have offered or any design details incompatible

with the broader design.

We will also usually make sure that the equipment weight, size, power, and other

utilities requirements are in line with our expectations. If they are not, we may need

to consider modifying the whole plant design to suit, or reject the item of equipment

as it is less favored when these knock-on effects are considered.


We do not wish to spend any more time on design at each stage than is necessary to

progress the overall design. Rule of thumb design is therefore the norm for process

plant designers.

We do, however, use more onerous but accurate heuristics as it grows increasingly

likely that some will actually build the plant.

RULE OF THUMB DESIGN

If you are working as a process designer, there will mostly (but not always) be a design

manual which will give the relevant rules of thumb for designing the items you are

being asked to design which encapsulates the company’s experience in the area.

Failing that, there will be a more experienced engineer in your department or

at least in your company who knows the rules of thumb. If there is neither a

manual nor such a person, this is a bad sign. You aren’t going to learn much in this

company.


If there is such a person, they may not be willing to share their knowledge. This

isn’t a great sign, but sooner or later the company will have to give you support if

they want you to do a competent job.

The more experienced engineer is often the world’s leading expert on the particular

job you have to do, as they don’t just know a way to design the plant, but they know

how to design the plant so that your company can build it. So be nice to him or her, as

they can teach you more than university ever did.

But what if you don’t have a manual or a Yoda? As well as Perry’s Handbook,

there are books which contain general rules of thumb—I give a couple of good ones

in the reading list at the end of the chapter. These are not going to be as good as the

experienced engineer, but they may well be more useful than attempting to use some

of the theoretical approaches they taught you at university.

There are some recent books offering “rules of thumb” generated by modeling and

simulation programs. Don’t use these. Proper rules of thumb come from experience

with multiple full-scale real-world plants, not first principles computer programs. First

principles design doesn’t work.

APPROACHES TO DESIGN OF UNIT OPERATIONS

First principles designDon’t ever do this in normal professional process plant design practice. Even if you

had enough data to allow you to design a unit operation from first principles, it would

be at best a prototype, and your employer would be the one offering the process

guarantee by making it part of the plant they were guaranteeing.

If you are designing unit operations for a living, you are not really a process

plant designer, so I will not cover this issue in much detail in this book about

process plant design. You probably have colleagues who have the necessary know-

how or, more likely still, a spreadsheet or program written by someone else which

encapsulates the know-how.

If you are asked to author such a spreadsheet or program, make sure it does reflect

the experience held in the company, that the program is without bugs, and verify its

accuracy across its range of operation by full-scale real-world experiments. It will have

fewer bugs if you write it in Excel rather than compiled code.

Design by simulation programDespite all that I have said about the use of simulation at whole-plant level, unit

operations are less complex than whole plants, and some suppliers now offer you


simulation program blocks which they have verified and tuned to match their real

equipment (though not validated for your application).

Companies which offer plants which are made of blocks of a limited number

of unit operations running on basically invariant feedstocks (as when, e.g.,

producing nitrogen from air) can produce model blocks which encapsulate much

empirical data.

In this way the most normal of normal process design activity can become

rather similar to building a Lego model. I am not, however, sure that we can

call this activity process plant design. It seems to me more like equipment

design, where a whole plant may be specified as a collection of standard

equipment.

Design from manufacturers’ literatureSince detailed design involves putting together unit operations you can actually buy,

manufacturers’ catalogues are a useful tool for selecting the unit operations which we

put into our plant designs.

Updated catalogues also frequently include new items of equipment we might not

have considered if we had not read the catalogue.

Back when I only designed plants for a living, reading through the pile of sup-

plier catalogues which had accumulated while I was engrossed in designing and

pricing the last plant was a very useful way to spend the time waiting to be allocated

my next job.

Nowadays these catalogues are more likely to be found on websites rather than in

hardcopy, and this is very handy in an academic setting, allowing us to bring realism

to our students’ designs without bothering manufacturers with enquiries from students

who are not actually going to purchase anything.

One thing which we find hard to recreate in the academic setting is

interactions with technical sales staff. Their detailed knowledge of their products and

their capabilities and limitations can allow plant designers to see new ways to design

ourselves ahead.

Civil engineers are, by the way, convinced that picking “gubbins” from

catalogues is all process engineers do for a living. This is a point they may make to

you when you tell them they can’t have the data they need to proceed with their

design for a couple of weeks. (The standard riposte to this is to tell them that

whatever you say, they will in any case specify the concrete wall they are going to

design to be 150 mm of concrete containing two sets of #10 M rebar.)

155How to Design Unit Operations

SOURCES OF DESIGN DATA

For the professional designer, the strength of sources of design methodology

information is generally as follows (in descending order):

Close to full-scale pilot plant trialThoroughly developed and validated tailored modeling program, directly based onmany full-scale installations of exactly the type and size of plant proposedSupplier InformationRobust rule of thumb direct from Chartered Engineer with lots of relevantexperienceRules of thumb from a book by Chartered EngineerDirect from Chartered Chemical Engineer with little relevant experienceFirst principlesGuessing by beginnerLess than thoroughly debugged and validated simulation and modeling programoutput

As far as sources of data required inputting to design methodologies is con-

cerned, strengths are generally as follows for feedstock and product qualities and

quantities:

Statistically significant ranges of values for the exact type and scale of plant envisagedStatistically significant ranges of values for the type of plant envisagedRanges of values for the exact type of plant envisaged, falling short of statisticalsignificanceRanges of values for the broad type of plant envisaged, with or without statisticalsignificanceGuessing by beginnerAs far as thermodynamic and other physical and chemical data is concerned, the test

of validity should be whether the data is intended by those generating it to be valid over

the range of physical conditions likely to be encountered under all reasonably

foreseeable plant operating conditions.

SCALE-UP AND SCALE-OUT

In theory, there is no difference between theory and practice. But in practice, there is.Yogi Berra

Theoretical types and scientific researchers fondly imagine that if a reaction works

in a conical flask, getting it to work in a 100 m3 reactor is a pretty trivial thing. If this


were true, there would be no chemical engineers. There is a lot more to chemical

engineering than chemistry.

There are two basic approaches to making a bigger plant. We can have a lot of

parallel streams of plants which we know to work at the given scale (scale-out), or we

can have a smaller number of larger but at least slightly experimental plants (scale-up).

What we need to know as engineers to carry out a scale-up exercise is the critical

variable or dimension. This variable is the thing we need to keep constant (or vary in

a predictable way) in order to get the process to work at the larger scale.

We might need to maintain the length, area, or volume of a process stage, or it

might be more complex, such as a number of theoretical plates for a distillation

column.

It should be noted that the key variable might change at different stages of scale-up

as the balance of effects varies. Scale-up by a factor of more than about 10 from even a

good pilot plant study should make us quite nervous as designers.

We might also need to ask the chemists to go back and make the reaction

work with less hazardous reactants or solvents, or in some other way restrict their

freedom to get them to offer a process which can be made to work in an

economically viable plant.

There is a lot to this subject, but the main thing to grasp is that something work-

ing in a 250 ml flask is no guarantee at all that it will be cost-effective, safe, or robust

in a 25 l vessel, let alone at full scale.

NEGLECTED UNIT OPERATIONS: SEPARATION PROCESSES

Many students leave university with knowledge of only 6�10 separation processes,

usually those most important in oil refining, and the knowledge they have of these

processes is mostly at best engineering science.

Processes based in chemistry are overrepresented in university courses, as are liq-

uid/liquid separations. The consequent lack of knowledge of options impoverishes

the designer’s imagination, and a lack of understanding of engineering design practice

prevents practical use of technologies.

Table 11.1 provides an overview of the most important separation processes,

arranged by phases separated. However, it is not exhaustive, does not go beyond

separations of two components, and does not mention the plethora of subtypes of the

technologies listed.

There is a book which fills this gap in the knowledge of professional plant designers,

which is not much used in academia: Couper (see “Further Reading” for details).


Table 11.1 Separation processesRelativeprice

Processrobustness

Safety concerns Separation principlea Product recoveryper stage

Contaminantremoval perstage

Gas/Gas

H M Rapidly rotating

equipment

Sedimentation M M

Adsorption M M Surface interaction VH

Distillation H L/M Flammable/toxic

vapors, heat

Differential

vaporization/condensation

L/M M/H

Filtration L H Physical exclusion ofoversize particles

H H

Gas/Liquid

Distillation H L/M Flammable/toxicvapors, heat

Differentialvaporization/

condensation

L/M M/H

Adsorption M M Surface interaction H VHStripping L H Flammable/toxic

vapors/liquids

Mass transfer from

liquid to vapor phase

M M

Filtration L H Physical exclusion of

oversize particles

H H

Gas/Solid

Adsorption M M Surface interaction H VH

Freeze-drying H M Low temperatures Sublimation H VHFiltration L H Physical exclusion of

oversize particles

H H

Elutriation M M Sedimentation M/H M/H

Cyclones M H Sedimentation M/H M/HDrying M H Heat Evaporation, usually

heat assisted

VH VH

Liquid/Liquid

Decantation L H Sedimentation M/H H

Extraction M M Mass transfer from one

liquid phase to

another

M H

Membranes M L/M Physical exclusion of

oversize droplets

H H

Chromatography VH L/M Differential affinity VH VH

Liquid/Solid

Crystallization M M Mass transfer from

solution to crystal

M/H H

Coagulation L M/H Destabilization of a

colloid

M M

Precipitation L H Exceeding solubility

limit

M/H M

Flotation M M/H Sedimentation effected

by reducing solid or

liquid density with

gas bubbles

H H

Ion exchange H M Differential affinity VH VH

Electrolysis H H Electricity Electrochemistry H H

Centrifugation M M Rapidly rotating

equipment

Sedimentation M/H H

Hydrocyclone M M Sedimentation M/H M/H

Magnetic

separation

H M VH VH

Extraction/

Leaching

M H Mass transfer from solid

to liquid phase

M/H M/H

Chromatography VH L/M Differential affinity VH VH

Membranes H M Physical exclusion of

oversize particles for

coarser membranes,

diffusion for RO

VH H

Electrophoresis VH L/M Electrochemistry plus

drag effects

VH VH

(Continued)

Table 11.1 (Continued)Relativeprice

Processrobustness

Safety concerns Separation principlea Product recoveryper stage

Contaminantremoval perstage

Solid/Solid

Classification M M/H Sedimentation/

adhesion/electrostatic

M/H H

Sublimation M H Sublimation VH VH

Magneticseparation

H M/H Differential magneticattraction

VH H

Real-world separators differ from mathematical/theoretical ones in ways which usually make any first principles design unworkable.aThese principles are major influencers of the separation process, but they are at best tools for analysis and partial understanding.

FURTHER READINGCoulson, J.M., Richardson, J.F., 1995. Chemical Engineering: Fluid Flow, Heat Transfer and Mass

Transfer. Butterworth-Heinemann, London.Couper, J.R., Penney, W., 2012. Chemical Process Equipment—Selection and Design. Elsevier,

Amsterdam.Green, D.W., Perry, R.H., 2007. Perry’s Chemical Engineers’ Handbook. McGraw-Hill, New York,

NY.Hall, S., 2012. Rules of Thumb for Chemical Engineers. Elsevier, Oxford.Sinnot, R.K., Towler, G., 2005. Chemical Engineering Design, Vol. 6. Butterworth-Heinemann,

London.Woods, D.R., 2007. Rules of Thumb in Engineering Practice. Wiley-VCH, Weinheim.












CHAPTER 12

How to Cost a Design

INTRODUCTION

Engineering . . . to define rudely but not inaptly, is the art of doing that well with one dollar,which any bungler can do with two after a fashion.

Arthur Mellen Wellington

Engineering is a commercial activity. Sufficient effort is put into pricing at each

stage of design to allow a rational commercial decision to be made as to whether to

proceed to the next stage, but ideally no more. Costing itself has costs.


Conceptual design is sufficient for what contractors would call a budget estimate of

costs. If you get a real budget estimate from a contractor, it will probably be accurate

to around 630%, as they have lots of data from equipment suppliers and genuine

knowledge of just what it costs to engineer and build plants.

Beginners without this information and experience can produce estimates out by

several hundred percent (almost always underestimates). They tend to leave out everything

other than the very core of the production process, have unrealistic ideas of the cost of

engineering and construction, no knowledge of the cost of engineering by other disci-

plines, and so on. Many of my students also seem to be willing to forgo profit, which is the

whole point of engineering. They certainly frequently forget to add it to their estimates.

Beginners tend to use exactly the same techniques and make the same errors if

asked for a more accurate costing. Professionals working in contracting companies do

a very detailed design and price all the goods and services required to supply it, con-

sider risks, margins, contingency, and so on (or, if they don’t work in a contracting

company, they ask a favor of someone who does).


Engineers have (of course!) quantified this into five classes of estimate as given in

Table 12.1. These are used by public bodies in the United States and worldwide (see

also the AACE Practice Guide in “Further Reading”):

THE BASICS

The most basic point of all is: if you aren’t considering price, you aren’t doing

engineering. Engineers consider the cost, safety, and robustness implications of every

choice we make at every stage of a project.

I have worked in a few places where technical and economic evaluation have been

split, and all have provided salutary lessons in why they should not be. Decision

making processes were very poor, and too easily swayed by fashion (yes, there is such

a thing in engineering!) or the whim of managers.

The degree of confidence you have in your technical design is the maximum

degree of confidence you should place in your costing. More usually we have to price

in all kinds of other risk factors to arrive at a robust pricing. So it isn’t just a question

of what the kit costs, risk needs to be priced.

You have process risks—the more novel the process, the greater the chance it will

underperform, or fail to perform at all. If your plant fails its performance test, your

company will probably be paying penalties every day until it is fixed at your com-

pany’s expense. You can buy performance bonds which insure process risk, but they

cost money, and the more novel you have been, the more they are likely to cost.

Then you have financial risks—overseas contracts can be subject to currency

fluctuations, and even home contracts can see significant inflation. If you have made

heavy use of some material subject to price fluctuation (which need be no more

exotic than stainless steel), things can cost a lot more than you expected.

There are political risks—countries can fall out with each other, industries can be

nationalized without compensation, wars break out and, closer to home, regulation

can disallow certain approaches, or make them—for example waste disposal—far

more expensive than you originally costed for.

Sensitivity analysis is the key to understanding these risks, and deciding how to

price them. You are unlikely to win a competitive tender if you price all of them in

to your offer at 100% probability. A guide to the kind of price which is reasonable

Table 12.1 Classes of cost estimateEstimate class Name Purpose Project

definition level

Class 5 Order of magnitude Screening or feasibility 0�2%

Class 4 Intermediate Concept study or feasibility 1�15%Class 3 Preliminary Budget, authorization, or control 10�40%

Class 2 Substantive Control or bid/tender 30�70%Class 1 Definitive Check estimate or bid/tender 50�100%


would be probability of occurrence multiplied by cost of occurrence. Many of your

competitors in a commercial situation will, however, undercut this value considerably.

In commercial practice you need to consider all of these factors, and produce a

price accurate to a few percent.

This price will need to be based upon a design which is optimized to meet the

client tender evaluation criteria: lowest price that meets the specification, lowest

whole-life cost, best net present value (NPV), fastest payback period—all affect every

aspect of competitive design.

In one way or another, all design is competitive. Even if you are doing an in-house

design, it needs to be the best design it can be against the evaluation criteria, and you can

rest assured that when it goes out to the engineering contractor, they will be redesigning

it as much as they are allowed to maximize their profit, and minimize their risks.

ACADEMIC COSTING PRACTICE

In order to decide if it is economic to proceed with a design we need a quick way to

estimate capital and running costs. The main plant items (MPIs)/factorial method is

almost always used in academia (though far less commonly in practice).

Douglas (see later section) offers a more sophisticated process which increases in

resolution as the project progresses, namely economic potential. This is, however, fre-

quently misused nowadays—the more rigorous stages he proposes are left out, with

the result that costing is reduced to the vestigial stub of a comparison of feedstock and

product costs.

Capital cost estimation by MPI/factorial methodWe cannot obtain supplier quotations for all of our equipment and engineering

services in academia as a professional would, so we need a standalone costing method-

ology for use in the academic setting.

Chemical Engineering departments worldwide seem to do more or less the

same thing.

First, we estimate the cost of MPIs, usually from cost curves: Timmerhaus and

Peters (see “Further Reading”) contains many of these curves. We then add factors to

account for things like operating pressure, special materials and so on, to the base costs

for the curves.

Having added up all the MPI costs, we calculate installation and other engineering

and construction costs as a percentage of MPI costs, using Lang factors such as those

to be found in Chapter 6 of Sinnot and Towler (see “Further Reading”). This allows

us to estimate the capital cost (capex) of the plant.

An academic criticism can be made of this near-universal academic approach,

enshrined in Sinnot and Towler, the bible of academic design practice. Lang factors

date back to the 1930s, and other more accurate factors have since been devised to

replace them.

165How to Cost a Design

Operating cost estimationIn academia, operating costs (opex) are usually estimated as a percentage of capital costs,

often a nominal 10%. It is actually possible to get a lot closer to professional practice

than this (even in a university setting) but very few in academia try, as far as I know.

Professionals estimate how much power, chemicals, manpower, capital, and so on

will be required to run the plant, and price these inputs at market rates. There is no

reason why even the greenest student cannot try this approach, though obviously they

will not be as good as an experienced professional. My experience in asking to students

to do this is that they are not as terrible at it is as you might think they would be.

Economic potentialEconomic potential (EP), as it is explained in Douglas’s Conceptual Design of

Chemical Processes, does start with a simple comparison of feedstock and product

prices, but rapidly advances to a far more sophisticated accounting of costs and bene-

fits than the standard MPI/factorial approach.

The approach makes a number of assumptions which mean that it is only applica-

ble to a subset of process plant types, but is in its appropriate setting superior in my

opinion to the MPI/factorial method.

The very sketchiest version of EP (intended only for use before any design has

been undertaken at all) is frequently nowadays the sole costing consideration in

academic “plant design” exercises.

Payback period, NPV, and so onA slightly more sophisticated financial analysis can be undertaken in an academic

setting, as well as in professional practice.

Payback period tells us how long it takes to get back our capex from revenues/

profits. NPV discounts future revenues and expenditure to reflect the fact that we care

less about our money in the future than we do about our money now, and also infla-

tion/interest rates on money.

NPV can incidentally be criticized, as large expenditures far in the future are auto-

matically thought fairly unimportant. This can be used to justify projects with very

high future decommissioning costs (such as, e.g., oil rigs and nuclear power plants) in

ways which green groups disagree with. Accountancy is not value free.

Sensitivity analysisEven though academic costing methodology is necessarily a bit flaky, we can firm

things up (or at least quantify our flakiness) with an honest sensitivity analysis.

Sensitivity analysis varies the costs and revenues which might apply to a system

and considers the shape of the curves obtained. If profitability falls off sharply around

your assumed costs and revenues, your process economics are not very robust.


I am personally not so bothered about whether students or other beginners get a

realistic price, as whether they know how good their price is—give me a range in

which the professional price lies, and a realistic estimate of where it is most likely to lie.

PROFESSIONAL COSTING PRACTICE

I spent most of the first five years of my career producing proposals for turnkey plants

for design and build contractors in the (ultra-competitive) international water industry.

I was pretty good at it by the end, and I used to win quite a lot of contracts for the

plants I designed and bid.

This was sometimes based on price and sometimes on technical merit. It isn’t

always about getting the lowest price on the table—it does usually help a lot, though.

I have been keeping my hand in in the intervening years and little seems to have

changed other than that we now make a lot more use of computers and external

design consultants that we used to.

Accurate capital cost estimationUsually, competitive bids are invited from potential suppliers for the various goods

and services used to construct a plant before a process contractor makes a firm offer

to an ultimate client. Three is usually thought a good number of bids to have for any

item. A smaller number means that there might be a limited number of places where

that item can be obtained, which is risky.

Bids are checked against the specification, to ensure that all which has been asked

for has been included (frequently not the case), and that the requested payment terms

and other contract conditions have been complied with (also frequently not the case).

Once bids have been standardized, prices are compared, and a supplier is selected on

an “or approved equal” basis.

These prices constitute firm offers by third parties to supply the item for a given

sum. They are not at this point estimates, they are guarantees to offer the goods for

the price quoted.

Enquiry documents need to be detailed enough to allow suppliers to understand

completely what is required both technically and commercially. If they are not, suppli-

ers may decline to quote, or may price in the uncertainty.

Purchasing companies will have their own terms, ultimate client companies will-

have theirs, and equipment vendors will have their own. It is frequently the case that

enquiry documents will ask for quotations based on a combination of client and

contractor terms, and vendors will offer their own terms in their offers.

This is not a trivial matter, and the differences in prices between alternative suppli-

ers can be less than the price implications of variation in contract terms. This issue

will need resolving to obtain a firm price basis.


If you work in a process contracting organization, you may well have access to

many such firm prices for exactly the kind of equipment you are pricing from previ-

ous jobs. The basis of your estimates can be very accurate indeed.

Bought-in mechanical itemsProfessional engineers price unit operations as one or more purchased items of

equipment (known as “bought in items,” i.e., physical plant bought as discrete items)

by sending enquiry documents to relevant equipment suppliers.

These prices usually have to have sums added to address the bits the various suppli-

ers have left out of their bids, so that they can be evaluated on a like for like basis.

They will probably also have sums added to reflect risk. For example, the fewer

potential suppliers you have, the greater the risk that prices will rise, or that your bit

of kit will not be available in time or at all.

Bought-in electrical itemsControl panels, aka Motor Control Centers (MCCs) can be bought as a discrete item

or along with electrical installation and/or software supply.

It will usually require input from an electrical engineer, and probably an element

of in-house design to be able to produce sufficiently detailed enquiry documents to

obtain reasonably accurate quotations for MCCs.

PCs, PLCs, DCS systems, or supervisory computers may also be bought as discrete

items or integrated with the MCC.

If anything, greater care needs to be taken to adjust bids, and to price risks associ-

ated with these bids, than it does with those for mechanical equipment.

Mechanical installationMechanical installers will usually supply (in addition to the skilled labor required to fix

and mechanically commission the mechanical bought in items) the pipework, bracketry,

supports, and so on required to make a working plant. They may also do detailed design

of pipework support systems, supply any nonspecialized valves, and so on.

These bids are at best only as good as the drawings the bidders have been issued

with, though they are less prone to underestimation and price escalation than electri-

cal installation bids.

Electrical installationSupply and installation of cables, emergency motor stop buttons, site lighting and

small power, and making connections from MCC to motors will normally be the

responsibility of a specialist contractor.

This element is possibly the most prone to underestimation by beginners. It is

important to issue sufficient information to installers to make sure that everything


needed has been accounted for and, ideally, the offer should be checked by an

in-house electrical engineer.

Software and instrumentationThis may be provided in-house by some combination of MCC supplier or installation

contractor. A specialist may be used to install and commission instruments, program

PLCs, and set up SCADA, DCS, remote telemetry, and such systems.

Whoever is doing it, great care has to be taken in pricing this element, as it is a

major source of cost overruns at construction stage, especially due to underestimation

of the number of inputs and outputs to the system.

Civil and building worksHow much is a ton of pumps?

Anonymous Civil Engineer

Civil engineering companies work on very tight margins, and tend to interpret their

communications very literally. They work from drawings, so you need to make sure

that anything issued to them for pricing is very clearly marked with respect to those

elements which you are willing to stand by later, and those which are indicative only.

Their pricing methodology is based on counting tons of stuff. Once they have

completed a design, they “take off” from their drawings how many tons of concrete,

steel, and so on are required. They are consequently usually in a hurry to get their lon-

gest lead time item (design) started, and will pressure you for the required information.

It is best to wait until you have a reasonable degree of certainty before issuing it, if

for no other reason than because civil engineering companies have a reputation for

being rather more litigious than other disciplines.

Civil and building costs are relatively easy to control as long as you have nailed

down the usual weasel words in civil engineering pricing documents (“unforeseen

ground conditions” for starters) during the initial stages.

Design consultantsNowadays, companies are increasingly using the services of design houses to carry out

design, particularly for specialist items.

If you are going to do this, you will need to price it in, and allow for the strong

possibility of requirements for additional design work later in the project.

This can come to a surprising amount of money. At the time of writing, the going

rate in the United Kingdom for an experienced process design engineer is d150 per

hour or so.


Project programmingProfessional engineers produce a schedule or program of events setting out the time-

scales for the key elements of the design/construction/commissioning phase and allo-

cating resources against each of the tasks required.

This allows pricing of those items whose costs are based entirely on their duration

of use (such as, e.g., hire of site cabins) as well as indicating how many hours will be

required for each discipline, and whether the company has the resource to handle the

project in-house, or will need to buy in (usually more expensive) external resources.

Man-hours estimationThe plant design engineer will have produced their estimate of how many hours of

each discipline it will require to do the job, but the discipline heads within a company

will also want to give their estimate of how long it will take their people to do it.

Since they are the ones who have to deliver the project, and the plant designer is

responsible for winning the work, discipline head estimates tend to be on the high

side, and plant designers on the low side. There should be some negotiation.

Pricing riskOnce you have prices for all the goods and services you need to make the plant, you

need to make sure that you have allowed money toward the chance that process,

financial, legal, political, or other risks go against you.

As well as adding sums to individual prices as previously described, you might do

this formally by buying a form of insurance known as a performance bond, which

usually costs a fraction of a percent of the complete contract value. You might add an

overall contingency, which is built into your price. Alternatively you might declare

the risk to the client, and include a prime cost (PC) sum which you would charge if

the possible adverse event materializes.

MarginsMargins vary greatly from industry to industry. Back when I was pricing water treat-

ment plants for a living in a very competitive sector, we were happy to get paid 22%

more than our bought in costs.

Some very sharp practitioners were bidding contracts at less than cost, by leaving

things out which had to be included later (under what are called variation orders,

VOs) at top dollar.

Generally, the less money there is swilling around in a sector, the tighter the mar-

gins will be, and the more sharp the practitioners.


Competitive design and pricingIt’s the only kind I know, and this book is based throughout on the assumption that

process plant designers are doing it for profit rather than fun (though it is fun when

you get the hang of it).

You can cut your margins of safety as far as you dare, you can negotiate with suppliers,

discipline heads, and financial directors at the pre-tender stage, but you can only get so

far by reducing your bought in cost and margins by either arm-twisting or charm.

The way to win better contracts more of the time is to design yourself ahead.

Don’t do what everyone else is doing, but a little less well, for a little less money—do

something better. That’s why process engineers get the big bucks.

You don’t need to be too radical to find all sorts of little ways to be a little bit cleverer

than the other guy, and if you find enough of them you can win work with decent

margins.

Much of it is to do with seeing the system working together as a whole and seeing

the full implications of making small changes. It’s all about system level design, the

subject of the next part of the book.

Accurate operating cost estimationWith a well-developed design, the contractor knows how many operator man-hours

are required, and has an idea of what each discipline costs an employer. They know

estimated chemical use, and can calculate expected effluent costs with the Mogden

formula. They have accurate estimates of hours run for motors, and can forecast the

price of electricity. They have maintenance schedules and costed spares lists obtained

from suppliers, and so on. The contractor should consequently be able to cost the

expected running cost for the plant to a high degree of accuracy.

FURTHER READINGAACE International, 2005. Recommended Practice no. 18r-97: cost estimate classification system—as

applied in engineering, procurement, and construction for the process industries 2005 AACEInternational. ,http://www.aacei.org/non/rps/17r-97.pdf..

http://www.reliabilityindex.com/manufacturer.Peters, M., Timmerhouse, K., 2002. Plant Design and Economics for Chemical Engineers. McGraw-

Hill, New York, NY.Sinnot, R.K., Towler, G., 2005. Chemical Engineering Design, Vol. 6. Butterworth-Heinemann,

London.


http://www.aacei.org/non/rps/17r-97.pdf

http://www.reliabilityindex.com/manufacturer





PART 4

High Level DesignThe previous section was a description of how to design the components and subsystems

of a process plant, but the ability to do this is not why process plant designers get the big

bucks. We are paid to produce a design integrated at a higher level than this, so that all

the subsections work together well.

I have identified three areas where a whole-plant understanding is most

important—process control, layout, and safety. This understanding has to be fed back

to the subsystem design covered in the last section, but these three areas have to be

addressed primarily at the whole-system level.

CHAPTER 13

How to Design a Process ControlSystem

INTRODUCTION

As Myke King (see “Further Reading”) has pointed out, much of what is taught in

Chemical Engineering courses under the heading of process control is out of date,

irrelevant, and impractical, with the result that most new process plant designers have

little idea of how to design the process control aspects of their plant.

What a plant designer needs to be able to do is to specify control loops based on

instruments and control actions which make the plant approximate steady state under

all conditions thought reasonably likely (or, to put it another way, within the design

envelope). In order to do this, standard control approaches for unit operations are an

excellent starting point.

Myke also advocates having sufficient consideration of process control issues to

build controllability into the design, an approach developed more fully by Luyben

(see “Further Reading”), albeit in quite a narrow field.

I see the rationale for this but I am not sure that Luyben’s formal and simplified

approach is the answer. Like so many elements of process design, academic approaches

laboriously solve problems which can be better solved by simpler intuitive means.

I can see the need for integrating process design and control, but I would go fur-

ther, including under this heading things which might not be thought of as process

control elements, such as hardware selection, hydraulic design for passive flow equali-

zation, integrated consideration of process control and safety elements, and the inter-

action both of operating and maintenance manuals/operators, and functional design

specifications/software. I give an example from my own experience of how this works

in practice at Appendix 1.

Once they start as practitioners, feedback to novice designers from commissioning

and control engineers or their attendance at HAZOPs will hopefully eliminate fea-

tures which lead to poor controllability, but I would like to give newbies more of a

head start than is presently usual.


Integration of process control and design by professionals is far more intuitive and

qualitative than mathematical. To quote Myke King:

It’s a difficult subject. I’ve learnt how to design control strategies instinctively. I’ve been askedon many occasions to document a methodology. I’ve got as far as “Work in the industry for40 years and you get the hang of it.”

Difficult it may be, but beginners definitely seem, in my experience, to need to be

given a place to start. Interactions with more experienced engineers will refine their

understanding, but what they need in the first instance is a way, as an absolute mini-

mum, to put the basics on their Piping and Instrumentation Diagrams (P&IDs). That

is what this chapter aims to provide.


At conceptual design stage, very little or no consideration needs to be given to process

control issues, unless the plant has some novel or very hazardous components which

are likely to present entirely new or very high-risk process control problems.

At the detailed stage of design, a fully thought out and instrumented P&ID needs

to be produced, and ideally, precise models of instruments specified. As a minimum,

realistic instrument choices and specifications should be produced. Instruments can be

expensive, and vary between manufacturers in their requirements and capabilities.

Perhaps more importantly, the number of inputs and outputs to the control system can-

not be determined unless the instrumentation has been thought through to this degree.

Modern instrumentation tends to be smart, with considerable onboard processing

power. We need to decide how we are going to use this. Are we going to have smart

instrumentation with dumb control, or dumbed down instrumentation with smart control?

And then there is the question of whether we are going to have a smart plant with

dumb operators or a dumb plant with smart operators? Plants tend to be smart nowadays,

but I have been asked to design a fully manual plant to be run by postdoctoral researchers.

The software for a dumb plant is defined in the Operation and Maintenance man-

ual, and for smart plants in the functional design specification (FDS). As most plants

have some smarts nowadays, a combination of the two documents will be required to

understand how the designer thinks the plant will be controlled.

OPERATION AND MAINTENANCE MANUALS

Operation and Maintenance (O1M) manuals are written for (almost) every process

plant, describing how it is to be operated and maintained, and how to troubleshoot

any problems which occur. They certainly should be written (and they should also be

read!).


They are to me largely a type of process control software since, on a fully manual

plant, they describe in detail the control actions which people will undertake to

achieve the things which a programmable logic controller (PLC) would do on a fully

automatic plant.

Most plants are, however, not fully automatic. There are automatic control actions,

and there are manual interventions. Some of these manual interventions are required

by law. For example, the UK Institution of Electrical Engineers Regulations requires

under certain circumstances (such as motor overheating) stopping the operation of a

motor in such a way as to require manual intervention to restart.

Some of these conditions are thought trivial enough to allow the system to auto-

matically restart itself via remote command. Some are thought dangerous enough that

the system forces someone physically to press a button before restart is possible and

the O1M manual tells them that they must to go and look at the kit before they

press the button.

So decisions have to be made about safe operation of the plant; and how software

and operating procedures will work together to ensure safety.

Control philosophies always, to my mind, make implicit assumptions about how

the plant will be operated, which it is better to include in the document, as another

reader may make different assumptions if they are not made explicit.

SPECIFICATION OF OPERATORS

The level of education and training of operators and their availability has to be speci-

fied to determine the degree of automation which a plant requires.

In choosing whether to have a highly automated plant, one needs to consider the

advantage of operators over instrumentation—operators can detect not just specified

conditions, but unspecified and unexpected conditions.

The more we expect our operators to do about the things they monitor, the high-

er their required level of skill and understanding needs to be.

A fully manual plant will need a high availability of highly experienced staff. A

fully automatic plant may need no permanent staff on-site at all, especially now that

we can access plant telemetry and system control and data acquisition (SCADA) sys-

tems remotely via IP technology.

AUTOMATIC CONTROL

We don’t build fully manual plants in the developed world nowadays. Computers are

too cheap and reliable, and operators too expensive (and human!), for routine opera-

tion activities to be best done by people alone.

177How to Design a Process Control System

Control is mostly done using a combination of PLCs, PCs, and high level control

system (distributed control system (DCS) or SCADA), though there may be a few

field-mounted controllers specified for a number of reasons.

We don’t make a lot of use of physical field-mounted PID controllers of the kind

still talked about in university process control modules, and the things most like them

which we do occasionally use have their own built-in control algorithms.

Process plant designers do not write the software for these controllers. We might

to some extent if we were commissioning or control engineers tuning the control

loops, but we would probably do the majority of that by plugging in a laptop and

pressing the “optimize” button on the manufacturer’s dedicated software.

Commissioning (while very important) is not the subject of this book. Process

plant designers need to know how to specify instrumentation and control hardware,

populate their P&IDs with these items, and write FDSs so that software engineers can

design and price their software.

Process plant designers need to have an idea of what neighboring disciplines do,

and what they need to do their jobs. We don’t, however, need to be able to do their

jobs; we have to be broad-brush people. We don’t sweat the details.

Specification of instrumentationInstrument engineers/technicians (aka tiffies) have their specialism, but we don’t need

to be one of them to specify instruments well enough to design a process plant. Table

10.8 should help you with this.

We should be willing to be corrected by a tiffy at more detailed design stages on

details of instrument choice, as we should by other specialists.

It is, however, unlikely that if we are experienced designers, our choice would not

have worked at all. The specialist’s choice might, however, work a little better (as long

as they fully understand what we want the instrument to do).

PrecisionPrecision in mathematics is (confusingly to engineering students) to do with what

engineers call resolution. When I tell my students off for “spurious precision,” this is

the sense in which I am using it.

For example, in the filter pretreatment example in Appendix 1, I say that we need

control of pH to within 0.1 pH units. This is not the same as saying that we need

control to within 0.10 pH units, which implies 10 times the (mathematician’s)

precision.

Precision in engineering is different, and is to do with repeatability and reproduc-

ibility. It is not to do with how close the measured value is to the true value (accu-

racy) or the smallest change in the measured value with the instrument can detect


(resolution). It is to do with whether the instrument will give me the same reading

against the same true value the next time I test it.

We might further split engineering precision into reproducibility and repeatability,

the first encompassing variability over time, and the second being precision under

tightly controlled conditions over a short time period.

(I received a few comments from my correspondents about these definitions, and

offers of alternatives, so it is fair to say that there is a problem with precision in the

language of precision. These are the definitions I choose to use for the purpose of this

book. To quote Humpty Dumpty: “When I use a word, it means just what I choose it to

mean—neither more nor less”).

In our specific example, pH probes require regular recalibration against standards

to maintain precision and accuracy. Over the period between calibrations, the accu-

racy (measured value for a given true value) varies. Eventually it is not possible to cali-

brate the instrument to give accurate readings against the standards, and a new probe

has to be substituted. There are gradual decreases in accuracy, precision, and response

time during the periods between new probe installations.

AccuracyAccuracy is to do with the gap between the true value and the value indicated by the

instrument. In the filter feed treatment example mentioned above, I need pH to be

controlled within the range around the set point 6 0.05 pH units, therefore my mea-

surement accuracy needs to be reliably at least this good.

Cost and robustnessInstrument precision and accuracy both tend to cost money. Very precise and accurate

(basically lab grade) instrumentation also tends to be less robust as well.

Lab instruments tend not to be suited to field mounting. We therefore tend not to

specify any more accuracy or precision than we strictly need, and we may take manu-

facturers’ lab test values for an instrument with a pinch of salt.

All instrumentation needs to have a purpose to justify its cost. It may be true that

“you can’t manage what you don’t measure” but it’s best not to measure things you

don’t need to manage.

SafetySafety-critical instrumentation requires a higher standard of evaluation than that

which only affects operability, or less important still, process monitoring without asso-

ciated control actions.

We might, for example, specify for a process or for a safety-critical reading the use

of redundant cross-validated instruments (in which the reading most likely to be cor-

rect is determined by a voting system).


Specification of control systemsThe P&ID shows graphically, whilst the FDS describes in words, what the process

designer would like the software to do. Turning these deliverables into code is the job

of the software engineer.

PLCs are the basis of many modern process plant control systems, with DCS or

SCADA supporting the control interface or HMI. The petrochemical industry uses

PLCs only for low-level distributed control functions, and prefers to use a combina-

tion of DCS and a supervisory computer for overall plant control. This allows for

their more sophisticated control functions, which are at least as often retrofitted by

control engineers as designed into the original system.

The true nature of the control system should be reflected on P&IDs and in

FDSs. We should not expect to see a local control loop and field-mounted control-

ler on a P&ID representing a loop which actually works via signals going out and

back via PLC. There are appropriate symbols in the British Standard to show this

correctly.

STANDARD CONTROL AND INSTRUMENTATION STRATEGIES

In this section I will break down process control systems into some commonly used

blocks, which should allow you to populate your P&ID and control philosophy with

the standard features which appear on almost every plant.

I will assume that you know what feedback, feedforward, and cascade control are,

but that the rest of your university module on process control was taken up with

mathematical software engineering stuff about transforms and algorithms. In twenty-

first century process control, signal processing is built into the box, and algorithm

writing is done by the software engineer, though they may well need input from the

process engineer to do with outcomes of control functions.

Commissioning and control engineers who straddle the divide between process

engineers and software engineers need a deeper understanding, but their jobs are very

little to do with process plant design. Process plant designers do, however, need to

understand what software and control engineers are going to need from them, so that

they can design in controllability.

Alarms, inhibits, stops, and emergency stopsProcess plant designers will need the assistance of electrical engineers to ensure com-

pliance with the IEE regulations and the various European directives which apply to

this area.

However, I have included this section because beginners usually do not understand

that all electrical equipment needs to be easy to switch off in an emergency, and very


frequently comes with safety features that switch it off automatically in a number of

potentially hazardous situations. These might include such things as motor winding

over-temperature, motor over-torque, fluid ingress, and so on.

It is frequently the case (and it may be a legal requirement) that the more hazard-

ous of these cases will be set by the electrical/software engineers such that they

require an operator to attend site to reset the “trip.”

Less potentially serious conditions may stop motor operation only while the state

is current or, if less serious still, may only prevent the motor from starting. Both of

these conditions might be called inhibition.

All of these conditions will usually be set to generate local alarms in software.

More serious ones may generate off-site alarms, or activate an alarm beacon on

site.

A design which has an excessive number of alarms should be avoided. If there are

too many alarms, operators will be subject to alarm flooding, and develop what is

known in health care as alarm fatigue and either ignore them or find ways to disable

them. So, how many is too many? The Engineering Equipment & Materials Users’

Association (EEMUA) suggests the following criteria (See Table 13.1):

It should be noted that commissioning engineers frequently disable alarms and

interlocks during the early stages of commissioning, but this should be a planned

aspect of a commissioning procedure, and suitable substitute safety plans should be

made.

Many of these signals, alarms, and interlocks will have to be handled by the control

system, and leaving them out of the control system specification, if that is the case, is

a classic beginner’s mistake leading to cost overruns down the line.

Many are, however, wired directly into the motor starter, which eliminates a

potential weak link in the chain. Hard-wiring is standard for safety critical interlocks.

European standards also require the provision of emergency motor stop buttons

immediately adjacent to motors. Resetting the emergency stop locally cannot inciden-

tally allow the drive to restart, but there has to be a trip to reset on the motor control

center (MCC) as well.

Table 13.1 EEMUA criteria for acceptability of alarm rate in steady state operationLong-term average alarm rate in steady operation Acceptability

More than one per minute Very likely to be unacceptable

One per two minutes Likely to be over-demanding

One per five minutes ManageableLess than one per 10 min Very likely to be acceptable

EEMUA Publication No. 191.


Chemical dosingThere can be many nuances to design of dosing pump systems dealing with liquids

which release gases on suction, leak detection, overpressure, cavitation, and so on, but

I will deal here with the most common issues.

Pump speed control

There are now digital dosing pumps like the one illustrated in Figure 13.1 with inte-

grated speed and stroke control on board, working from digital inputs originating in a

flowmeter and pH probe.

However, it is still common to control a piston diaphragm pump’s motor speed

with a 4�20 mA signal from a flowmeter in the stream to be dosed. This is known as

flow pacing, and when used in conjunction with stroke length control as described in

the next section, it can give very accurate (6 0.1 pH units) pH control.

However, some engineers use simple speed control proportional to the difference

between measured pH and set point. This is okay, but not as precise as flow-paced

stroke control unless the flow into which we are dosing is always constant.

Figure 13.1 Memdos Smart LP: stepper motor controlled dosing pumps offering smoother almostcontinuous dosing. Copyright image reproduced courtesy of Alldos.


A more old-fashioned way to do this simple type of control is to send pulses to

the pump at a frequency corresponding to the desired stroke frequency. Commonly

available pumps and pH controllers can usually handle both pulsed or 4�20 mA con-

trol signals.

Pump stroke length control

A 4�20 mA signal from a dedicated pH controller can be input to a suitable flow-

paced dosing pump to control stroke length, giving a robust two-variable control of

chemical dose (Figure 13.2).

Actuated valve controlThere are still some plants being built in which an actuated valve is used to add

chemicals by gravity into a mixed tank but, to put it very politely, this is a bit old

hat nowadays. Control loop time is long, chemical flow control is pretty rough,

and the homogeneity at the point of pH measurement is questionable.

Figure 13.2 Memdos E ATE: mechanically actuated diaphragm dosing pump with inverter con-trolled motor and actuator/servomotor for automatic stroke length adjustment. Copyright imagereproduced courtesy of Alldos.


Compressors/blowers/fansPositive displacementPositive displacement blowers need similar control systems to positive displacement

pumps (see later), though the compressible nature of gases makes these systems a little

more forgiving than their liquid equivalents.

Centrifugal

Centrifugal compressors (Figure 13.3) are more efficient at large sizes than positive

displacement blowers, but they are more difficult to control. They are therefore quite

often favored where there are high fixed flows.

They are, however, capable of variable output. Back when I started as an engineer,

we used to do this on single stage compressors with variable position inlet guide

vanes, and on multistage compressors with inlet throttling valves, but nowadays

inverter control is usually favored.

Surge conditions—in which too low a flow causes a sudden powerful reversal of

flow—have to be avoided, and this is usually achieved via a control valve in a bypass

back to the compressor suction. There may be one of these valves for each compres-

sion stage, going back to the inlet of that stage.

Such valves may also be used to control flow through the compressor if inverters

are thought too expensive. As inverters get relatively cheaper all the time, I would

predict that this will eventually become an obsolete approach.

Figure 13.3 CAD representation of centrifugal compressor control.


Distillation

There are many processes and measured variables which we might consider—which

of the 120 possible permutations will we pair to which? There is a lot to this, and for

more detail I recommend Myke King’s book, a chapter of which is dedicated to what

he considers a broad outline of the subject. I have illustrated in Figure 13.4 his basic

suggestion, which is essentially pressure controlled.

Figure 13.4 CAD representation of distillation column control.


FiltersBackwash control

The standard methodology for differential head control of particulate filters is that

accumulated dirt is removed by reversing flow through the unit, known as backwash-

ing (Figure 13.5). This is done periodically on the basis of a number of criteria:

• Differential pressure (almost always)

• Time since last backwash (almost always)

• Queueing/hierarchy of wash initiation (very frequently if there are filters in

parallel)

• Outgoing turbidity (fairly rarely)

• Outgoing particle size analysis (very rarely)

Each filter will therefore need measurement of incoming and outgoing pressure,

via separate instruments or a differential pressure instrument. There will usually also

be a timer in PLC software (or less frequently nowadays in MCC hardware).

Queuing, if required, will be handled in PLC software.

Figure 13.5 CAD representation of backwash control.


Solids removal efficiency can be measured online via turbidity or particle size anal-

ysis. Turbidity measurement is reasonably cheap and robust, though it adds complexity

which is usually redundant. Particle size analyzers are very expensive and fragile kit,

and best avoided if at all possible.

There will need to be a backwash pump, whose output flowrate is crucial to effec-

tive backwashing. An associated flowmeter is therefore required. The backwash flow

needs to be high enough to effect dirt removal, though not so high that the filter is

damaged, but the acceptable range of flows is usually fairly broad.

It is therefore usually the case that that the commissioning engineer sets this flow

by throttling a manual valve or setting a range of inverter outputs, and thereafter it is

just monitored and returned to commissioning values by maintenance staff if required.

We may, however, sometimes specify a more sophisticated system with temperature-

dependent flow control of the backwash pump to ensure a constant mass rather than

volumetric flowrate during backwashing.

Chemical cleaning control

Membrane filters often require a chemically enhanced backwash (CEB) in addition to

simple backwashing (Figure 13.6). While the control of this is quite sophisticated, based

on analysis of trends in differential pressure across the membranes compared with origi-

nal condition and the condition after the last backwash, the instrumentation and avail-

able control actions are basically the same as for simple backwashing. The modifications

to backwash frequency, cleaning chemical type and strength and so on, which are insti-

tuted in response to declining membrane performance, are usually manually initiated.

Figure 13.6 CAD representation of chemical cleaning control.


The CEB system’s tanks, dosing and centrifugal pumps, flow control, and so on

are controlled as described in their respective section of this chapter. If a heated back-

wash is used, there is a control loop which modulates the output of a process heater

in response to a temperature measurement. This loop may well be critical—such sys-

tems (most notably the very expensive membranes themselves) are often made of

polymers which can be damaged by even quite moderate excessive temperatures.

Fired heaters/boilers

Fired process heaters have to account for variation in composition of feed, and varia-

tion in pressure in the case of gaseous fuels. It may also be the case that the heater

feed flow cannot be controlled, as its feed is the product of another process and can-

not be economically stored.

One way around this is the dual firing option shown in Figure 13.7 above. The

heater duty is set above that of the greatest expected yield of uncontrolled feed gas,

and a second fuel is added as required to top up the heater output to the duty required

based on the temperature of the fluid being heated.

This is a very complex area, which Myke King discusses in some detail in his

book “Process Control: A Practical Approach” from which the above diagram is

taken.

TC042

A B

TC045

B

TO PROCESS

DWG.No.: THIS DRAWING

05

FC055

Figure 13.7 CAD representation of fired heater/boiler control.


As he explains there, boiler control is essentially the same as fired heater control

except that the control is via steam header pressure rather than based on heated fluid

temperature. He also draws attention to the difference in control requirements for

fixed duty (baseload) boilers and the assist (swing) units which are used to control the

steam pressure.

Heat exchangers

Heat exchangers (Figure 13.8) are usually designed to have temperature sensors in

both process and service streams going in and coming out. It is possible to vary the

service flowrate to control process stream temperature. Tighter control is, however,

given by bypassing the process side with a control valve in the bypass—when this

valve is operated, the temperature changes almost immediately.

Figure 13.8 CAD representation of heat exchanger control.


PumpsDry running protection

Many pump types are damaged if they run without liquid for any length of time, so

control loops are used to prevent this and are a standard feature of pump configuration

(Figure 13.9). The most common variant is an interlock between pump running or

starting and the level in a tank feeding the pump, such that low level in the feed tank

inhibits pump running and/or starting.

Ultrasonic, radar, hydrostatic, or float type level sensors are most commonly used

to provide the level signal. Float switches are very cheap, ultrasonic are good for non-

contact measurements of aggressive liquids and powders, and hydrostatic or radar

types are good if there is likely to be significant foaming. An interlock can be wired

directly into the motor starter from the sensor, or control can go via PLC.

Figure 13.9 CAD representation of dry running protection (of P14/15 by LITx0420).


No flow protection

Many modern flowmeters can detect empty pipe conditions, and this can be used as a

secondary measure to prevent dry running (Figure 13.10). Alternatively, flow switches

can be used for this duty. This is sometimes hardwired into the starter. Going via the

control system is, however, probably best so that a timer can be placed in the loop to

help commissioning engineers to prevent nuisance trips from transient conditions,

especially in the case of flow switches.

Over-temperature protectionMost electric motors come with thermistors incorporated in the windings, so that

drives can be stopped automatically if they are getting too hot. This safety critical

interlock is usually hardwired into the starter.

Figure 13.10 CAD representation of no flow protection (by FITX028).


Pumps: centrifugal

Centrifugal pumps (Figure 13.11) are not immediately damaged by being operated

against a closed valve (though they can overheat in fairly short order and even water

pumps have been known to suffer steam explosion) but throttling their suction can

cause immediate cavitation.

Their output can be controlled by running them against a control valve in the

delivery line, or by a bypass valve returning output to pump suction, though I person-

ally prefer to use the more efficient inverter control.

Flow delivered by a rotodynamic pump is inversely proportional to system pressure

(though Q/H curve shapes differ), but a flowmeter on the delivery side can be used

to control the degree of actuated control valve opening or inverter frequency to accu-

rately deliver the desired flowrate against variable delivery pressure.

Centrifugal pumps offer no resistance to reverse flow (and can generate unwanted

electricity if run backwards), and protection is sometimes put in to address this,

though nonreturn valves are usually thought sufficient protection if their (approxi-

mately 10%) backflow prevention failure rate is acceptable.

Figure 13.11 CAD representation of centrifugal pump control.


Pumps: positive displacement

Positive displacement pumps (Figure 13.12) are quickly damaged by being run against

a closed valve on the delivery side, and are not usefully controllable by throttling on

either side. You can control them to some extent with a valve on a bypass to suction,

but very accurate control can be given using an inverter drive.

I would still recommend the use of a flowmeter to modulate the bypass valve posi-

tion or inverter frequency. Though delivered flow is largely independent on backpres-

sure, wearing parts in these pumps may cause delivery volumes to drop during the

intervals between servicing.

These pumps are normally protected from damage caused by valve closing or other

line blockage with a pressure relief valve, placed between the pump and the first valve

downstream.

Figure 13.12 CAD representation of positive displacement pump control.


Pumps: dosing

The most modern digital dosing pumps have on-board integrated stroke speed con-

trols, driven directly by digital signals. Figure 13.13 shows the pressure relief valve

(PrV2) which protects against pump damage in the event of line blockage, as well as

the pressure sustaining valve and pulsation damper which remove flow pulsations.

Piston diaphragm pumps commonly come with two 4�20 mA inputs for control.

One controls motor speed, the other stroke length.

Solenoid pumps are far simpler (and cheaper); their solenoid produces a stroke for

every pulse of power sent to it.

There is more detail on this in the “Chemical dosing” section of this chapter.

Figure 13.13 CAD representation of a positive dosing pump control.


Tanks

Though not favored by those who like elegant solutions, and ideally to be made as

small as possible, buffer tanks (Figure 13.14) make for a flexible and robust plant

design.

Breaking the plant into sections ending/starting with a buffer or break tank is a

solution favored by most commissioning engineers, as it makes it easy to commission

the plant in sections.

Pumped flows can be ramped up and down, based on levels in the feed and/or

delivery tanks, in such a way as to maintain either a fairly constant tank level, or a

fairly constant flow.

In either case, rapid changes in flowrate and on/off control of pumps should be

avoided. Smoother operation is normally better operation. Level sensors such as ultra-

sonic and hydrostatic types, which measure level continuously, rather than trip at a

threshold level are therefore preferred.

Ultrasonic level indicator controllers can handle this control function well using

their on-board electronics.

Figure 13.14 CAD representation of break tank filling and emptying control.


Valves

Figure 13.15 shows how an on/off actuated valve can be used to control the level in a

tank. There are some details of the nature of control valves which are not commonly

explained in university courses and which I will cover in the sections which follow.

Figure 13.15 CAD representation of actuated valve control.


Rotary actuators—modulating duties

Globe and other valves used in a modulating duty require multiple controlled turns to

go from open to closed, and consequently require multiturn rotary actuators, which

are most frequently electrically driven (Figure 13.16).

Butterfly valves go from open to closed in 90� of shaft rotation, and there are

“quarter-turn” actuators used to operate them. Such actuators may be electrically or

pneumatically driven.

Figure 13.16 Rotary actuator. Copyright image reproduced courtesy of AUMA.


Linear actuators—open/closed duties

There are linear actuators which are used in a vertical orientation with globe and

other rising spindle-type valves, usually in on/off applications (Figure 13.17).

Valve positioner/limit switch

Valve positioners and limit switches tell the system when the valve has reached a cer-

tain position, so that it can be reliably driven to a certain degree of opening

(Figure 13.18). This gives a positive indication that the valve has in fact reached the

Figure 13.17 Linear actuators (painted red). Copyright image reproduced courtesy of Ascendant.

Figure 13.18 Valve positioner/limit switch. Copyright image reproduced courtesy of Ascendant.


desired position. This is not a guarantee of a given valve headloss or throughput, but

it does increase the accuracy of operation considerably.

FURTHER READINGAli, R., 2013. Keep it Down. The Chemical Engineer, November 2013. Available at ,https://www.

tcetoday.com/B/media/Documents/TCE/Articles/2013/869/869alarms.pdf..King, M., 2010. Process Control A Practical Approach. Wiley, Chichester, UK.Luyben, W.L., 2011. Principles and Case Studies of Simultaneous Design. Wiley, Chichester, UK.


https://www.tcetoday.com/&sim;/media/Documents/TCE/Articles/2013/869/869alarms.pdf



http://refhub.elsevier.com/B978-0-12-800242-1.00013-X/sbref1


CHAPTER 14

How to Lay Out a Process Plant

INTRODUCTION

It has been estimated that 70% of the cost of a plant is affected by the layout, and its

safety and robustness are if anything even more strongly dependent on good layout.

This area is the most notable omission from modern chemical engineering

courses, and it is also the reason why I came to write this book. In intro-

ducing (perhaps reintroducing) professional engineering design to the University

of Nottingham’s degree courses, I found that we used to teach an entire degree

module on this subject.

J.C. Mecklenburgh, a former engineering practitioner who taught at the

University of Nottingham, wrote an IChemE book to accompany the module

which, until recently, was still in use at many other universities. My offer to pro-

duce an updated version of that (now out of print) book led to the book you

are reading being commissioned. So this is arguably the most important chapter of

this book.

I have borrowed heavily from Mecklenburgh’s approach throughout this chapter.

I wish I had space to include more of his content, as his book is hard to get hold of

(at the time of writing, Amazon had a few copies priced at d460 each), and is mostly

still current professional practice. I am presently producing an updated version of

Mecklenburgh’s book, which should be available by 2017 at the latest.

There are a number of aspects to the more or less complete omission of plant lay-

out from today’s university courses. There has more generally been a loss of drawing

and visual/spatial skills from chemical engineering courses. Many universities (to the

extent that they use drawings at all) accept Google Sketch or MS Visio sketches which

bear no resemblance to engineering drawings. They restrict even these drawings to

Block Flow Diagrams (“BFDs”) or half-baked approximations of Piping and

Instrumentation Diagrams (P&IDs).

Then there is the tendency to reward abstraction and purism in academia. Lecturers

are taught on our teaching courses that the “extended abstract” is the manifestation of

the highest sophistication of student submission. Professional researchers need little

encouragement to think that realism isn’t just unnecessary, it isn’t very clever.

Not being very clever is the worst thing of all in a university. There is also the

usually unvoiced suspicion which many scientists have that artists aren’t very clever.


It is often thought axiomatic by academics that there is a hierarchy of different “intel-

ligences” thus:

logical/mathematicallinguisticsocialspatial intelligenceOf course, all of these are really just secondary issues. The main reason academics

don’t teach layout and drawing is because they can’t do these things themselves. They

only seem trivial to people who haven’t tried them.

Having carried out design exercises with a wide range of audiences, I can confirm

that only the most excellent academics (usually professors) produce plant layouts as

good as their best students. Plant layout requires flexing those mental muscles which

universities usually do not exercise. So how do novice designers make a start? Let’s

start with some principles.

GENERAL PRINCIPLES

We need to consider layout in sufficient detail from the very start of the design pro-

cess. Even at the earliest ages of design, attempting to lay plant out will throw up

practical difficulties.

Layout is not just a question of making the plant look pretty from the air (or more

usefully from public vantage points). The relative positions of items and access routes

are crucial for plant operability and maintainability, and there are more detailed site-

specific considerations.

The three key elements which have to be balanced in plant layout are cost, safety,

and robustness. Thus, wide plant spacings for safety increase cost and may interfere

with process robustness. Minor process changes may have major layout or cost conse-

quences. Cost restrictions may compromise safety and good layout.

The layout must enable the process to function well (e.g., gravity flow, multiphase

flow, net positive suction head (NPSH)). Equipment locations should not allow a haz-

ard in one area to impinge on others, and all equipment must be safely accessible for

operations and maintenance.

As far as is practical, high cost structures should be minimized, high cost connec-

tions kept short and all connection routing planned to minimize all connection lengths.

Layout issues at all design stages are always related to the allocation of space

between conflicting requirements. In general, the object most important to process

function must have first claim on the space. Other objects must fit in the remaining

free space, again with the next most important object being allocated first claim on

the remaining space. The constraints always conflict, and the art of layout lies in bal-

ancing the constraints to achieve an operable, safe, and economic layout.


Layout most obviously affects capital cost, since the land and civil works can

account for 70% of the capital cost. Operating costs are also affected, most obviously

through the influence of pumping or material transfer cost and heat losses, but more

subtly in increased operator workload caused by poor layout.

The layout must minimize the consequences of a process accident and must also

ensure safe access is provided for operation and maintenance. Things further apart are

less likely to allow domino effects from explosion, fire or toxic hazards, but it is likely

that lack of space means that complete mitigation of fire, explosion, and toxic risk

will not be achievable by separation alone.

Layout starts by considering the process design issue of how the equipment items

function as a unit and how individual items relate to each other.

For example, the individual items in a pumped reflux distillation unit should be close

together for effective fluid flow and minimal heat loss. The condensers and drums should

be near ground level to reduce the cost of associated structures, but the drums must

be elevated to provide NPSH for the pumps. Such relationships may sometimes be identi-

fied by a study of the Process Flow Diagram (PFD) or P&ID, but not all will be as obvious

as this example. This is where a General Arrangement (GA) drawing, experience, and

judgment become vital to find and balance the physical relationships in the layout.

There are many other factors to consider. Equipment needs to be separated in

such a way that it can be safely accessed for maintenance, for other safety reasons such

as zoning potentially explosive areas, and to avoid unhelpful interactions.

Exposure of staff to process materials needs to be minimized. Access to areas han-

dling corrosive or toxic fluids may need to be restricted. This may require the use of

remotely operated valves and instruments located outside the restricted area.

In a real-world scenario we will have a site or sites to fit our plant on to. Different

technologies will have a different “footprint.” They will have a required range of

workable heights and overall area. They may lend themselves better to long thin lay-

outs or more compact plants.

There may be a choice to be made between, for example, permanently installed

lifting beams, davits, and so on, more temporary provision, or leaving the eventual

owner to make their own arrangements.

There are no right answers to these questions, but professional designers will need

to have given these issues sufficient consideration to be able to argue that they have

exercised due diligence. In the United Kingdom, the minimum extent of this due dili-

gence is specified in the Construction Design and Management (CDM) regulations.

Layout does not require complex chemical engineering calculations but it does require

an intuitive understanding of what makes a plant work, commonly known as professional

judgment. If it were more common, we might call it “engineering common sense.”

These qualities cannot be taught formally, but must be acquired through practice.

It is, however, possible to start the learning process in an academic setting by design

practice, though judgment will mostly be developed in professional practice.

203How to Lay Out a Process Plant

Mastery comes only from learning from experienced practitioners and by testing

one’s own ideas rigorously. The best way to do this is by listening to those who build,

operate, and maintain the plants you design for 15 years or so, and adopting their

good ideas (no matter how embarrassing the learning process might be during layout

reviews).

Generally speaking, the most economical (and easiest to understand/explain to

operators) way to lay out a plant is for the process train to proceed on the ground as it

proceeds on the P&ID, with feedstock coming in on one side, and product out of the

far side of the site or plot. There may, however, be arguments for grouping certain

unit operations together in a way which does not correspond with the P&ID order if

the site is on multiple plots with any degree of separation.

Land is cheap in some places and expensive in others. Tall plants are generally

more favored away from human habitation, but some places don’t mind big ugly

plants if they come with jobs attached. Such places might also be happy to have quite

dangerous processes close to human habitation, as we were before we could afford to

be as fussy as we are now.

FACTORS AFFECTING LAYOUT

The main things to consider in laying out a plant or site are the same as they were in

Mecklenburgh’s day: Layout is, in short, the task of fitting the plant into the minimum

practical or available space so that each plant item is positioned so as to balance the

following competing demands.

Cost• The capital and operating costs must be affordable (e.g., placing heavy equipment

on good loadbearing ground).

• The plant must be capable of producing product to specification with the practical

minimum levels of operation, control, and management.

• Regular maintenance operations should be capable of being performed as quickly

and easily as is practical. Units should be capable of being dismantled in situ and/or

removed for repair.

• The plant must be arranged so as to promote reasonably rapid, safe, and economi-

cal construction, taking into consideration staging of construction/length of deliv-

ery period.

Health/safety/environment• Safe and sufficient outgoing access for operators, and incoming access to close to

units for fire fighters needs to be designed in.


• Operating the plant must not impose unacceptable risk to the plant, its operators,

the plant’s surroundings, or the general population.

• Operating the plant must not impose excessive physical or mental demands on the

operators. Manual valves and instruments, for example, need to be easily accessible

to operators.

• Design out any knock-on effects from fire, explosion, or toxic release.

• Avoid off-site impact of noise, odor, or visual intrusion, mainly by moving such

plant away from boundary and communities, and presenting nice offices and land-

scaping to public view.

• The plant, whether enclosed in buildings or outdoors should not be ugly through

uncaring design, but should blend with its surroundings and should appear as the

harmonious result of a well-organized, careful design project reflecting credit on

the designer and plant operator.

RobustnessThe plant and its subcomponents must be so arranged as to operate and make its

product(s) as specified.

• Process requirements (e.g., arranging plant to give gravity flow).

• The plant must be designed to operate at the planned availability and should not

be subject to unforeseen stoppages through equipment failure or malfunction.

• Consider how the plant might be expanded in future, and allowing space and con-

nections to do so easily.

• Consider access to commissioning resources in layout.

Site selectionThe site layout must provide a safe, stable platform for production over a period

which may be measured in decades. It is essential to define, early in the design

process, if the site is to be used by the designer or others for a single plant or if several

plants are to be installed either now or in the future.

If future plants are planned, some assessment of future development is needed. It might

be that space needs to be reserved, road networks planned, and major utility distribution

expanded to serve the new plants. When a site specification is drawn up, the site layout

aims to make the best use of all features of the site and its environs, for example:

• Site topography

• Ground characteristics

• Natural watercourses and drainage

• Climate

• External facilities

• Water, gas, electricity supplies


• Effluent disposal services

• Transport of people and goods

Due care and attention also needs to be given to the effects of the plant on the site

surroundings, especially:

• Housing

• Hospitals, schools, leisure centers

• Other plants

• Forests, vegetation

• Wildlife

• Rivers and groundwater

• Air quality

If we have a number of candidate sites which we need to choose between, the fol-

lowing factors should be considered at a minimum:

• Desired layout of the proposed complex

• Cost, shape, size, and contours of land/degree of leveling and filling needed

• Loadbearing and chemical properties of soil

• Drainage: natural drainage, natural water table, and flooding history

• Wind: direction of prevailing winds and aspect, maximum wind velocity history

• Seismic activity

• Legacies of industrial activity like mine workings, chemical dumps, and in-ground

services

• Ease of obtaining planning permission

• Interactions with both present and planned future nature of adjacent land and

activities

Many of these are environmental factors. We might split them into three catego-

ries: natural, man-made, and legislative.

Natural environmentWeather varies greatly from place to place. Singapore has lightning on average 186

days a year. Cherrapunji in India had 2.5 m of rain over two days in 1995. Death

Valley has air temperatures of up to 57�C, and Antarctica down to 289�C. A gusting

wind speed of 300 mph was once recorded in Oklahoma City. There may also be

sandstorms, earthquakes, tsunamis, floods, snow, hail, fog, and so on to consider.

Might our pipes freeze, or might they be softened by foreseeable ambient tem-

peratures? How much rainwater might we need to handle? What earthquake and

wind loadings do we need to specify? All need to be considered from the start.

Well-designed plants are site-specific: process plant designs cannot be cut and

pasted from one location to another, as a recent TCE article (see “Further Reading”)

suggests is occurring in South East Asia.


Man-made environmentAt the detailed design stage, we will need to consider the possibility of effects on and

interaction with surrounding plants, installations, commercial, and residential

properties.

In the United Kingdom, this is likely to involve interaction with regulators such as

UK Department for Environment Food & Rural Affairs (DEFRA), the Environment

Agency (EA), and Planning Authorities.

Regulatory environmentThe likely effluents, emissions, and nuisances (gaseous, solid and liquid as well as noise

and odor) and any abatement measures need to be considered at the earliest stage.

It should not be assumed that regulatory authorities will allow any release to envi-

ronment, or sewage undertakers allow any discharge to sewer without discussion with

them.

As well as the question of simple permission, there will be the question of emission

quality, which will set the size and cost of on-site treatment, or the ongoing bill for

third-party treatment. Not all effluents can be economically treated on site, and third-

party costs can be very significant, so this needs to be considered at the earliest stages

of design.

In the United Kingdom, third-party costs can be predicted using the Mogden

formula:

C5R1M 1Vm1V 1Bv1OtB

Os1

StS

Ss

R is Reception charge

V is Primary treatment charge (also referred to as P)

M is Treatment charge where effluent goes to a sea outfall

Bv is Biological treatment charge (also referred to as B1 and Vb)

Vm is Preliminary treatment charge for discharge to outfalls

B is Biological oxidation charge (also referred to as B2)

S is Sludge treatment charge

Os is Chemical oxygen demand of settled sewage

Ss is Suspended solids concentration in crude sewage

UK readers can download a Mogden formula calculator free of charge here:

http://www.wrap.org.uk/content/mogden-formula-tool-0

It should be borne in mind that the regulations which cover releases to environ-

ment always become more stringent over time. Future-proofing might be considered.

If you are not going to include additional plant, you need to at least allow space for

such plant.


http://www.wrap.org.uk/content/mogden-formula-tool-0

PLANT LAYOUT AND SAFETY

The IChemE book “Process Plant Design and Operation” (see “Further Reading”)

contains the following suggestions about safety implications of plant layout:

Incompatible systems should be separated from each other; humans from toxic fluids; corro-sive chemicals from low grade pipework and equipment; large volumes of flammable fluidsfrom each other and from sources of ignition; utilities should be separated from process units;pumps and other potential sources of liquid leakage should where possible not be locatedbelow other equipment to minimize the chance of a pool fire. (This is particularly importantwith fin-fan coolers where air movement may fan the flames)

The Health Safety Environment (HSE) states that the most important aspects of

plant layout affecting safety are for our designs to:

. . .prevent, limit and/or mitigate escalation of adjacent events (domino); Ensure safety withinon-site occupied buildings; Control access of unauthorized personnel and Facilitate access foremergency services.

So the advice is fairly consistent. A well-run Hazard and Operability (HAZOP)

should pick up any issues in this area, but designers should not go in to a HAZOP

with ill-considered layouts which will require extensive modification. HAZOP is not

supposed to involve redesign. Safety studies should be carried out as part of the design

exercise, as described in the next chapter.

As well as these guiding principles, there are many detailed considerations, such as

siting dangerous materials as far as practical from populations and control rooms, con-

sidering plan operability and maintainability, safe access and loading/unloading of

deliveries and collections, and so on.

Regulatory authorities including DEFRA, the EA, and Planning Authorities

(or such equivalents as may exist in other countries) may want to place restrictions

on the design with respect to siting, distance of certain structures from people,

height of certain structures, size of inventory of specified substances, releases to envi-

ronment, and so on. These issues need to be considered as soon as possible in the

design process. Certain types of plants, especially those likely to be covered in

Europe by Control of Major Accident Hazards (COMAH) legislation will need

special attention.

There are quite a number of codes of practice and guidance notes with respect to

plant layout as well as many specific codes for things like installations handling liquid

chlorine. A good place to start with these, especially for UK readers, is the HSE site

(see “Further Reading”). The Dow/Mond indices are also explained there—these can

be used at an early design stage as rules of thumb to give outline guidance on equip-

ment spacing. I include more international references in the next chapter, which is

specifically on safety.


The general principles designers most need to bear in mind are Inherent Safety

and Risk Assessment.

Mecklenburgh gave the following principles for separation of source and target:

• Large concentrations of people both on- and off-site must be separated from

hazardous plants.

• Ignition sources must be separated from sources of leakage of flammable materials.

• Firebreaks are wanted, and are often best provided by a grid-iron site layout.

• Tall equipment should not be capable of falling on other plant or buildings.

• Drains should not spread hazards.

• Large storage areas should be separated from process plant.

• Central and emergency services should be in safe areas.

He also produced the tables of suggested separation distances to be found in the

appendixes. These are of course for guidance only, and should be subject to the appli-

cation of engineering judgment.

PLANT LAYOUT AND COST

As far as capital cost is concerned, the further apart things are, the more piping and

cable is required to connect them. Things further apart also require more steelwork

and concrete, land and buildings to support and contain them. Smaller plant often

costs more than less space-intensive plant, but land costs money too.

With respect to operating costs, things further apart have higher fluid transfer

headlosses, higher cable power losses, and higher heat losses from hot and cold ser-

vices. It also takes more time to go from one part of a larger plant to the next.

There will be a balance to be struck between capital and operating costs, as

designers need to think of how every item of equipment will be maintained, how

motors and other replaceable items will be removed and brought back, and how

vehicles required by operational and maintenance activities will access the plant

during commissioning and maintenance activities as well as during normal

operation.

A few cost saving guidelines are as follows:

• Buildings should be as few in number and as small as practical.

• Gravity flow is preferred, and failing that pump NPSH should be minimized.

• The number and size of pipes and connections should be the minimum practical.

• Save space and structures wherever possible (consider safety!).

• Group equipment where practical and safe.

• Make multiple uses of structures, buildings, and foundations.

• Make full use of the height available.


• Consider column locations when laying out equipment in a building/choosing

building type.

• Don’t bury services under buildings.

• Storage tanks can be made of welded steel, bolted glass on steel panels, site

cast concrete, preformed concrete, plastics, and composite materials. All

should be considered, and have implications for layout with respect to con-

struction access.

• Ground conditions can affect economics of concrete tankage—it might most eco-

nomically be fully in-ground, fully out, or intermediate depending on the water

table and how hard it is to dig, etc.

• Ground conditions can also affect civil costs of locating heavy structures on a site.

Put them where the ground is good.

• Structure loading can interact with ground conditions—heavy structures can have

a low loading by being shorter and wider. This can be in tension with the process

design if a tall thin structure is required.

Having considered the most practical issues, we need to remember that we may

need to make our plant acceptable to the public (and their proxies, the planning

authorities) if it is to be built. It matters what it looks like.

PLANT LAYOUT AND AESTHETICS

Aesthetics can be very important to engineering designers, but it is more or

less absent from engineering curricula. Aesthetics falls within the province of

philosophy, or maybe psychology, which is bad news for any hope of coming

up with any definitive answers. Philosophers have yet to find a definitive answer to

any of the questions which they have been pondering for a couple of thousand

years now.

Personally, I like the look of process plants, and I think that great big flames

coming out of the top of flare stacks look very cool. However, this is a minority

opinion (though it is one shared by Ridley Scott, who incorporated the appearance

of the Wilton chemical plant near his boyhood home into his film Bladerunner).

Most people don’t like the appearance of process plants. Neither do they like how

they sound and smell, the associated vehicle movements, or the effect on their

house prices.

Architects can help us with aesthetics, though this may come at a price: here’s a

sewage pumping station designed by an architect for the London 2012 Olympics

(Figure 14.1).


This will usually be another issue settled by negotiation. Generally speaking, we

would like our buildings to be unadorned big metal sheds, and planners (and the gen-

eral public) may ideally like them to be ancient oasthouses saved from demolition by

being lovingly rebuilt on our site to contain our process plant.

This seemingly straw-man example actually comes from my past experience—I

recall one job where, after negotiation, we ended up with a row of token cowls of the

sort seen on oasthouses on a big tin shed, more than a hundred miles from the nearest

real oasthouse.

All of this aside, we will need to care about aesthetics, because planning author-

ities do (and some would say that good engineering is rarely ugly). The cost

implications of building something to the standards required where we will attract

NIMBY (“Not in My Backyard!”) resistance might make it more economical to

build it where people care more about jobs.

Architects will know the rules under which planners have to work, and the ones

who know how big a brick is are our best source of information on how to satisfy the

planners at least cost. The artist/philosopher type of architect will be far less helpful

to us than the other kind. It might be an idea to ask any architect you are considering

using how big a brick is (215 mmL3 102.5 mmW3 65 mmH is the right answer)

and save yourself a lot of heartache.

Practical architects may also help us to produce a plant which is a more pleasant

and efficient place of work. Staff morale is important—some even think it improves

performance.

Figure 14.1 London 2012 Olympic park pumping station. Copyright image reproduced courtesy ofthe Olympic Delivery Authority.



Conceptual layoutConceptual design aims to do enough work on layout to ensure that the proposed

process can fit on the site available, and to identify any layout problems which need to

be addressed in more detailed designs.

From a regulatory point of view you need to establish whether enhanced regula-

tory environments like COMAH or Integrated Pollution Prevention and Control

(IPPC) are going to apply to the plant, and design your layout to suit.

There are a number of issues which might be neglected at early stage design by

beginners.

In which direction is the prevailing wind?Upwind/downwind positions can matter, for example, pressure vessels should not be

downwind of vessels of flammable fluids, toxics should not be upwind of offices/per-

sonnel, and cooling towers need to be as far away as possible from anything which

would interfere with airflow, and so arranged as not to interfere with each other.

Indoor or outdoor?You also need to decide on whether you want indoor or outdoor plant. Good light-

ing, ventilation and air conditioning, protection for excessive noise, glare, dust, odor,

and heat help staff to do their work well, so indoor is often the most comfortable for

operators. Indoor is more secret, easier to keep clean, and indoor equipment usually

has a lower IP rating, making it cheaper to buy. Outdoor plant is, however, usually

best for toxic and flammable vapor dispersal, and buildings cost a lot of money.

Construction, commissioning, and maintenanceDesigners need to consider all stages of the plant’s life, and specifically vehicle access

and space for removal and laydown of plant and subcomponents like heat exchanger

tubesheets.

There may also be a requirement for tankers and temporary services for mainte-

nance, commissioning, and turnaround activities. This should be identified and

accommodated early in design.

Maintenance activities also need to be covered—instruments and plant requiring

regular attendance and maintenance should be reachable by short, simple routes from

the control room. (This is particularly important with batch processes, which often

require a lot of operator intervention.)

Where equipment is to be maintained in situ, space needs to be left for people and

tools to reach equipment for inspection and repair, the lifting gear required, and lay-

down for parts, and the design needs to consider accessing different levels in the plant

via ladder or staircase, and how to get tools to the working level.


Where it is to be maintained in the workshop, space should be allowed for people

and tools to reach equipment for inspection, disconnection and reconnection, the lift-

ing gear required for removal and replacement, and loading/unloading onto transport

to workshop or off-site.

In either case, the working area should be designed to be safe with respect to

access, lifting, confined space entry, electrical and process isolation, draining, washing,

and so on.

Items requiring regular access for operation, maintenance, or inspection should

not be in confined spaces or otherwise inaccessible.

Materials storage and transportThe size of on-site materials storage and transport facilities will be determined in the

first instance by practical issues of import and export from site of feedstocks, products,

and waste material. It should be noted that there may be a choice of road, rail, and

water transport to be made. Larger volumes of transport may be best handled by rail

or water transport, if practical.

The process’s requirement for storage and transport facilities will need to be mod-

erated by statutory and commercial standards and codes of practice, as well as planning

authority requirements.

There is commercially available software that allows the designer to overlay a vehi-

cle turning circle over the road layout to check that the roads are suitable for the pro-

posed use, and I offer guidance in the following section on suitable road and turning

circle sizes which can be used in the absence of such software.

Materials storage and transport considerations will form part of the Hazard

Assessment (HAZASS), and will probably need to be at least justified and possibly

reconsidered as part of planning and permitting applications.

The provision of utilities such as steam, compressed air, cooling, and process water

and effluent treatment facilities needs to be considered at the earliest stage, as their

design needs to be well integrated with that of the whole plant.

Emergency provisionMecklenburgh gives relevant dimensions for fire appliances to allow designs of

suitable roads, hardstanding, etc. to ensure suitability of access provision for emergency

use. To summarize, roads need to be at least 4 m wide, suitable for 20-ton vehicles

with turning circles of at least 21 m. About 5 m of hardstanding needs to be provided

5�10 m from buildings and items which might require firefighting.

SecurityAs soon as equipment is on-site, we will need a site fence to protect company and staff

property from theft, and prevent unauthorized access for safety reasons. We may want,


in some cases, to design out features which will shelter protestors at the site entrance,

or make plant buildings amenable to occupation.

Central servicesAdmin, welfare facilities, labs, workshops, stores, and emergency services need to be

well-sited, and should feature on designs from an early stage.

EarthworksEarthworks may be required to shield tanks of dangerous materials or controls from

the site fence. If we are close to communities, we might also want to consider bored

teenagers with air rifles. I once had a site where we had to put bulletproof shields on

all emergency shutdown switches visible from the fence, to protect them from target

practice, and another where we had to erect sight screens during commissioning to

stop them shooting at anyone in hi-viz.

Conceptual layout methodologyProfessional engineers and companies have their own ways to doing this, but

Mecklenburgh’s method for initial plot layout is okay (if perhaps a bit OTT) and goes

roughly as follows:

• Generate initial design, sizing, and giving desired elevations of major equipment.

• Carry out initial HAZASS (see next chapter) or apply MOND index, consider all

relevant codes and standards.

• Produce plan layout of plant based in this data and the suggested spacing in this

chapter (I cut out scale shapes from paper and arrange them on a big sheet of

graph paper before I go to CAD for this).

• Question elevation assumptions, consider and cost alternative layouts.

• Produce simple plan and elevation GAs of alternatives without structures and floor

levels.

• Produce more detailed plan drawing based on decision for last stage.

• Use this drawing to consider operation, maintenance, construction, drainage,

safety, etc. Consider and price viable looking alternative options.

• Consider requirement for buildings critically. Minimize where possible.

• Produce more detailed GA in plan and elevation based on deliberations to date.

• Carry out informal design review with civil engineering input based on this drawing.

• Revise design based on this review.

• HAZASS the product of the design review. Determine safe separation distances for

fire and toxic hazards, zoning, control room locations, etc. Consider off-site effects

of releases.

• Revise design based on HAZASS.


• Confirm all pipe and cable routes. Informal design review with electrical engineer

would be helpful.

• Multidisciplinary design review considering ease and safety of operation, mainte-

nance, construction, commissioning, emergency scenarios, environmental impact,

and future expansion.

• Reconcile outputs of design review and HAZASS, taking cost into consideration.

• If they will not reconcile, iterate as far back in these steps as required to reach

reconciliation.

Detailed layout methodologyThe detailed design stage for plants of any significance should include a number of

formal safety studies as well as less formal design reviews which should address layout

issues as well as the process control issues which may form the core of such studies.

If it is identified that the site will comprise a number of plots, interactions between

these plots and with any existing ones on the site and those on surrounding sites need

to be considered at detailed design stage.

Mecklenburgh’s detailed design procedure bringing together plot designs (as out-

lined in last section) into a site-wide design for multiplot sites is approximately as

follows:

• Compile the materials and utilities flow sheets for piping and conveyors as well as

vehicle and pedestrian capacities and movements on- and off-site.

• Lay out whole site, including areas for the various plots, buildings, utilities, etc.

• Use the flow sheets to place plots and processes relative to each other, bearing in

mind recommended minimum separation distances, sizes, and areas.

• Add in services where most convenient and safe from disasters.

• Next, place central services to minimize travel distance (but considering safety).

• Now consider detailed design of roads, rail, etc. keeping traffic types segregated,

and maintaining emergency access from two directions to all parts of the site.

• Identify and record positional relationships between parts of the plant/site which

need to be maintained during design development.

• HAZASS site layout, with special attention to the possibility of knock-on effects.

• Single discipline design review (Chemical; Electrical; Civil; Mechanical) of design

and construction functions, which should critically review the design from the

point of view of each discipline.

• Multidisciplinary design review. The various disciplines should critically examine

the design with respect to: Hazard containment, Safety of employees and public;

Emergencies; Transport and Piping systems; Access for Construction and

Maintenance; Environmental Impact including air and water pollution; and Future

expansion.


• If there is still more than one possible site location at this point, the candidate sites

can be considered in the light of the detailed design, and one selected as favorite.

Reviews at this stage may be undertaken in a 3D model in the richer industries

such as oil and gas, but many industries are still doing it with 2D hard copy drawings.

For constructionAfter site purchase, detailed design to optimize the site to its chosen location can be

undertaken. Mecklenburgh has a number of optimization steps earlier in the process,

but premature optimization is unwise, so I have dropped them.

Mecklenburgh has another stage of design review and optimization for the “for

construction” phase, which basically involves gathering very detailed data on the site,

the market for the products, and so on and testing the design assumptions from previ-

ous stages for a good match to the real site. He then recommends repeating the hazard

and design reviews, culminating in consideration of the plant with its wider

surroundings.

FURTHER READINGAnon, Undated. Plant layout. Health and safety executive, ,http://www.hse.gov.uk/comah/sragtech/

techmeasplantlay.htm..Eades, J. (2012) It Couldn’t Happen Here. The Chemical Engineer 857, November 2012, pp. 26�28.Mecklenburgh, J.C., 1985. Process Plant Layout. Institution of Chemical Engineers, London.Scott, D., Crawley, F., 1992. Process Plant Design and Operation: Guidance to Safe Practice. Institution

of Chemical Engineers, London.


http://www.hse.gov.uk/comah/sragtech/techmeasplantlay.htm

http://www.hse.gov.uk/comah/sragtech/techmeasplantlay.htm




CHAPTER 15

How to Make Sure Your Design IsReasonably Safe and SustainableINTRODUCTION

Why is Health Safety Environment (HSE) the number one concern? Process engi-

neering (especially water process engineering) saves far more lives than the best medi-

cal practitioners ever will, but we can do harm on an industrial scale as well.

Back when I applied to be a chartered engineer, the only aspect of professional

practice in which experience had to be demonstrated in all applications submitted was

“safety aspects of process plant design and operation.” I consider the subsequent

removal of this requirement to be a mistake, as you can become a chartered chemical

engineer now without having experience in this area. You cannot, however, become

a good engineer without this experience.

The worst doctor who ever lived might have killed a few hundred people over a

long period of time. Bad engineering could kill tens of thousands of people in a day.

The most pressing argument for the prime importance of safety issues is the more

or less universal ethical one that people should not die or be injured so that we can

make money. The argument for avoidance of environmental degradation is weaker.

Many societies are willing to put up with a degree of environmental degradation in

order to industrialize and develop, just as the industrialised West did. The IChemE

metrics reflect the engineer’s views on this, which are a fair bit more rational than the

views of some environmental pressure groups and anticapitalist protesters.

WHY ONLY REASONABLY?

Engineers make decisions on the basis of more or less formal cost benefit analyses. We

know that as we try to move toward perfect safety and sustainability, each incremental

improvement becomes progressively more expensive.

This fact is accommodated by UK and European law by the terms “as low as rea-

sonably practicable” (ALARP) and the even uglier acronym SFAIRP (so far as is reason-

ably practicable), which define the required standards of safety. These terms set a limit

on how far we have to go. We are not required to make plants any safer if the cost of

an incremental increase in safety is grossly disproportionate to the benefit gained.


So society tells us at least how safe they would like our plants to be through leg-

islation. There may, however, be conceivable situations in which our interpretation

of our moral obligations as professional engineers requires us to set a higher

standard.

In the United Kingdom and Europe in general, there is legislation which requires

higher levels of scrutiny of health, safety, and environmental aspects of a design under

specified circumstances. For UK designers, the Environment Agency’s “netregs” web-

site and the HSE’s are very good places to start looking at the requirements in more

detail. The most important aspects for process plant designers are as follows:

Control of Major Accident Hazards (COMAH) legislation requires that businesses

holding more than threshold quantities of named dangerous substances “Take all neces-

sary measures to prevent major accidents involving dangerous substances . . . Limit the conse-

quences to people and the environment of any major accidents which do occur.” There are tiers

within the legislation which impose higher duties on companies holding greater quan-

tities of these materials. Plant designers need to consider whether their proposed plant

will be covered by this legislation at the earliest stages.

Control of Substances Hazardous to Health (COSHH) legislation requires risk

assessment and control of hazards associated with all chemicals used in a business

which have potentially hazardous properties. Consideration of the properties of che-

micals used as feedstock, intermediates, and products is a basic part of plant design.

Inherently safe design requires us to consider these issues at the earliest stage.

There is a lot of similar legislation worldwide, and I would recommend the US

Center for Chemical Process Safety’s publications intended to assist process plant

designers in addressing safety issues (see “Further Reading”).

Plants which are more safe or sustainable than society requires are unlikely to be

built in the normal run of things. I have seen a few cases where this has been done

for marketing purposes by companies with enough spare cash not to worry too much

about the costs, but mostly plants are built to make stuff in a cost-effective, safe, and

robust manner.


We should consider inherent safety (designing out risks) at the very earliest stages of

design. Chemists are notorious among chemical engineers for devising process chem-

istry optimized for their batch/bench-scale processes, and for having a slightly gung-

ho attitude to safety issues.

Their greater tolerance of hazardous chemicals is understandable, as they work

with far lower quantities than process plants contain. Their bench-scale chemistry is,

however, rarely optimal for full-scale production.


CONCEPTUAL DESIGN STAGE

It would be unusual to go out of our way to design in sustainability, but we always

design in safety from the very start. This tends in earlier design stages to be informal

and instinctive, and progresses to more formal studies later in the project.

Formal safety studiesWe aren’t going to be able to do a formal Hazard and Operability (HAZOP) at the con-

ceptual design stage due to lack of project definition and documentation, but we can and

should carry out less formal Hazard Identification (HAZID) and Hazard Analysis

(HAZAN) studies as well as other industry-specific types of formal study appropriate for

this job.

It is very important to determine as soon as possible whether the plant is going to

come under enhanced regulatory regimes such as IPPC and COMAH. These will

have a major impact on plant design development construction and operation costs,

and may rule out certain locations and approaches entirely.

Inherent safetyRather than controlling hazards, we should design them out of our process from the

very start. Inherent safety is a way of looking at our processes in order to achieve this.

There are four main keywords:

MinimizeReduce stocks of hazardous chemicals (Trevor Kletz called this intensification, which

others later confusingly used to mean something else entirely).

SubstituteReplace hazardous chemicals with less hazardous ones.

ModerateReduce the energy of the system—lower pressures and temperatures generally make

for lower hazards.

SimplifyKISS! (Keep it simple stupid)

Kelly Johnson

Don’t design plants you don’t understand, and especially don’t pile safety features

one on top of another instead of solving the root problem.

Note that the principles of inherent safety are applied at conceptual design stage to

the proposed process chemistry.

219How to Make Sure Your Design Is Reasonably Safe and Sustainable

In the situation where we are being given the process chemistry by a product

development team, we need to consider whether they have considered the constraints

of full-scale operation. Are the selected reactants, solvents or process conditions the

most inherently safe ones? If they are not, and we are in a position to influence the

process chemistry, can we get the chemists to rethink the chemistry?

In the more common scenario where the technology/process chemistry is bought

in from a third party, can we choose another bought-in process which is more inher-

ently safe?

Human factorsSome of the worst accidents ever were caused by what might be called human error,

or more correctly by plants which were not designed with real operators and man-

agers in mind.

The IChemE are very keen on this issue, and in their latest discussion document

on the future of chemical engineering it is stated that “The crucial role of human fac-

tors in process safety will also shape the institution’s approach to process safety.”

There are lots of interesting books on the subject of how people interact with pro-

cess plants—Trevor Kletz’s are very readable.

I personally learned a lot by commissioning plant, watching, and training opera-

tors. In summary:

• Think about the limits of human attention

• Think about the limits of human physical capabilities

• Specify minimum competence of operators required

• Design your plant so that it is easy to do the right thing and hard to do the wrong thing

• Design your plant so that even if the wrong thing is done, disaster does not ensue

• Design your plant so that it is physically impossible to do truly disastrous things

• Design in controlled operation of your plant with a combination of operating pro-

cedures and control philosophy

• Consider carefully limits on access to operating software

Ideally we should, as a profession, build a knowledge-base of how difficult plant

situations were handled successfully in the past—being good is more than just not

being bad! The IChemE used to keep an accident database, but no longer. When I

ask my students if they have heard of Flixborough or Seveso, very few have.

Those who cannot remember the past are condemned to repeat itSantayana

User-friendly designTrevor Kletz identified a related concept to inherent safety, which addresses human

factors he called user-friendly design. There are a number of additional principles and

suggestions which are mostly amplifications of the four principles of inherent safety.


Tolerate errorsThere will always be errors in plant operation. If such errors readily lead to disaster,

your design is neither inherently safe nor robust. Design your plant to handle these

events, ideally passively.

Limit effects/avoid knock-on effectsIf there is a chance of hazardous events in plant operation, their effects on people and

environment must be minimized by designed features. It is especially important to

minimize secondary effects caused by damage accrued by the initial event. Note that

this applies only to hazards which it has not been possible to design out—it does not

contradict the four main keywords. For example, if tanks containing flammable liquids

have a weak seam around the roof, the lid may blow off, but the tank will not rupture

spilling the contents.

Make incorrect assembly impossibleBecause what can happen will happen, and if the effect of incorrect assembly is signif-

icant hazard, it needs to be designed out.

Making status clearObvious visual clues as to the status of plant help prevent accidents. To borrow Trevor

Kletz’s example, commissioning engineers use blanking plates to shut off process

lines—a variety of such plates known as a spectacle plate make clear from a distance

the status of a line.

Ease of controlControllability is a desirable feature of process plant designs in its own right, though

the safety case is perhaps the strongest justification.

DETAILED DESIGN STAGE

There will be formal safety reviews during detailed design, such as HAZOP, electrical

equipment zoning, and other industry specific analyses.

Note that formal does not mean quantitative-qualitative judgments by professional

engineers will very likely be the best way to pick up all significant risks until design

for construction stage. Quantitative methods applied before those late stages are both

overkill and a waste of resources.

These formal methods are the subject of the next part of the chapter.


FORMAL METHODS: SAFETY

In increasing order of rigor we have HAZID, HAZAN, and HAZOP. We also have

what Mecklenburgh calls “Hazard Assessment,” (let’s call it “HAZASS”) a design

review/risk assessment exercise focusing on safety aspects of layout, which falls some-

where between HAZAN and HAZOP.

HAZIDThe first step in management of risk and hazards is to identify potential hazards. This

is the purpose of HAZID studies.

Personal safetyBroadly there are five classes of personal safety hazards:

Physical/mechanicalSlips, trips, and falls; confined spaces; noise; burns, cuts, and strikes; heat/cold stress/

dehydration.

BiologicalRelease of hazardous organisms.

ChemicalRelease of hazardous chemicals.

ElectricalIncluding static electricity.

PsychosocialPoor plant design and management can cause physical and mental health problems for

workers. Many of these are potentially lethal to individuals, so consideration of them

should not be beneath a process plant designer’s notice.

HAZASSThis is a kind of design review, so it is best done in collaboration between a number

of engineering disciplines. The Mecklenburgh methodology is as follows:

• Define the process design with drawings (Piping and Instrumentation Diagram

(P&ID), General Arrangement (GA), Equipment datasheets, etc.).

• Identify sources of failure and vulnerable targets, and try to design out hazard by

removing failure mode or, failing that, path to target.


• Identify parts of plant containing dangerous materials which would be hazardous

to release even if no failure mode is identified.

• Estimate the frequency of release and potential rates and amount of leakage.

• Evaluate the potential consequences of release on targets.

• Adjust layout and/or design and repeat assessment until consequences of loss are

acceptable.

• Make emergency response plans based on the residual risk, and revise layout

accordingly.

HAZOPI will base this section on what I teach in academia, which is quite a bit more realistic than

what is usually taught there. The difficult thing about HAZOP is not in any case grasping

the methodology; it is being able to imagine the consequences of failure states, work effec-

tively as part of the HAZOP team, and stay on task (and awake!) throughout the procedure.

It would be usual for new engineers to be trained before attending a HAZOP,

usually a one-day course for simple attendance at a HAZOP, which would normally

offer a fair amount of practice at participating in mock HAZOPs. There are also lon-

ger courses for HAZOP chairs, and experienced chairs are often brought in from out-

side to lead company HAZOPs.

If you would like to explore HAZOP further than the outline I cover here, I

would recommend the late Trevor Kletz’s books on the subject (see “Further

Reading”) which are as readable and entertaining as they are informative.

Process plants are complex, and even the most experienced engineer cannot tell at

a glance all the ways in which the parts might interact. HAZOP is a formal technique

which allows us to consider how the plant we have designed operates under a number

of sets of operating conditions.

This is especially telling in an academic setting where steady state conditions may

have been overemphasized—there is no steady state in the real world, and to the

extent that we approximate it, it is a product of good process control.

Consideration of the construction, commissioning, decommissioning, start-up, and

shutdown phases should be incorporated into the HAZOP.

Commissioning may involve the use of chemicals not used during normal opera-

tion (e.g., for pipe pickling), the production of noise, dust, odor, and fumes, heavy

traffic loads with knock on effects like mud and dust transfer offsite, as well as a

requirement for temporary storage, office, welfare, and sanitary facilities.

During start-up and shutdown, commissioning and maintenance activities the sys-

tems responsible for safe operation may be absent. Is our plant safe—not just under

our expected operating conditions, but under the reasonably foreseeable conditions at

all stages of its life?


HAZOP and other similar tools allow us to systematically evaluate this question.

In a university we can only approximate the professional practice, but I will flag up

those important areas where we have had to lose an important aspect of the real-

world procedure in order to teach it in an undergraduate chemical engineering

course.

We will take a set of engineering drawings and supporting documentation, and

apply a basic HAZOP methodology to identify possible safety and operability issues.

The documents we will consider will be a P&ID, GA drawing, functional design spec-

ification, and a set of datasheets.

In professional practice, we would have a larger set of documents available to us,

and we would have a multidisciplinary HAZOP team. The inclusion of electrical,

software, and mechanical engineers as well as operating and commissioning specialists

would maximize the chance that all impacts are considered. In this academic exercise,

we will inevitably miss some of the issues these other disciplines would have raised,

but we are learning a methodology, rather than carrying out an actual HAZOP.

The basic HAZOP method is to inspect the P&ID one node at a time, and to per-

mutate parameters (flow, pressure, composition, temperature, etc.) with guidewords

(more, less, no, reverse, other) in order to identify hazard (to health and safety) and

operability issues (which might impact on profitability or the environment).

Nodes are usually selected as sections of the P&ID surrounding and including a

unit operation, encompassing those process lines which are most likely to be affected

by the unit operation, or to affect it.

There are a number of roles in a real HAZOP team which require specialist train-

ing, but I simplify this in academia to just two fixed jobs: a Chair to keep things on

track, and a Scribe to note down your findings.

Professional HAZOP procedure1. Core team: Chair, Scribe, Process Engineer, Control Engineer plus other engi-

neering disciplines and specialists in operation, commissioning, etc.

2. Consider as a minimum four scenarios: Start-up, Steady State Operation,

Shutdown, and Maintenance.

3. Go through P&ID one node at a time using guide words (NONE; MORE OF;

LESS OF; PART OF; MORE THAN; OTHER) for each parameter (flow; pres-

sure; temperature; component/impurity; phase; viscosity, etc.).

4. Make no assumptions. Chair should be asking “how do we know that . . ..” to

break all assumptions.

5. Record all deviations identified.

6. Identify changes to plant or methods which make deviation less likely or protect

against the consequences.


7. Decide if cost of changes is justified.

8. Agree justified changes, agree who is responsible for them.

9. Produce action list with responsibilities.

10. Follow up to ensure action has been taken.

ReportingThe conditions leading to these potential problems and the issues themselves are listed,

alongside the actions proposed to mitigate them, on an action report similar to that

used in a professional context (Figure 15.1).

In the academic exercise, the column identifying who is responsible for making

sure the corrective action is done is omitted. In the real world, this is very important.

Things which are everyone’s responsibility are no one’s.

In addition to P&IDs, the GA is an important tool to help put the cause/conse-

quence scenarios identified by the team into a plant context and visualize the possible

hazard location. When a consequence is identified, the P&ID does not show which

other items are in the vicinity and might be affected. The GA allows the team to

make a rapid judgment of the potential for knock-on effects.

Similarly, if a Quantitative Hazard Analysis is required, the hazard contours can be

drawn on the layout to show where unacceptable hazard levels are imposed on equip-

ment and access areas.

HAZOP REPORT SHEETName Title Role Sign Project title

Project No.

ELD No.

Sheet

Date

No. Guideword/parameter

Possible cause Consequence Action Personresponsible

Date to becompleted

Completionsignature

Figure 15.1 HAZOP action report sheet.


Professional HAZOP is a lengthy and expensive procedure not to be undertaken

lightly because of the unavoidable cost of the rigor which gives its results their value.

It requires quite a number of senior engineers, with prior training and experience in

the technique. Computer-aided design (CAD) tools are, however, becoming available

to emulate the procedure, which might offer real assistance to the HAZOP team.

HAZANHAZAN is an initial screening technique which has a number of variants. The general

principle is that a table is drawn up for the various hazardous events which might

conceivably happen on a plant, with likelihood of occurrence on one axis and severity

of outcome on the other, and the product of likelihood and severity is called risk.

Dow and Mond indexes are commonly used to rank hazards.

As with HAZID, this is done in accordance with the principles of Process Safety:

major hazards are considered as a priority.

Risk matrixA common approach to risk assessment and hazard analysis is the risk matrix. The

underlying idea is that acceptability of risk is a product of how likely a thing is to hap-

pen, and how bad it would be if it did (Tables 15.1 and 15.2).

These are combined to produce a risk matrix as in Table 15.3.

Table 15.1 Risk matrix: categorization of likelihoodCategory Definition Range (failures per year)

Certain Many times in system lifetime .1023

Probable Several times in system lifetime 1023 to 1024

Occasional Once in system lifetime 1024 to 1025

Remote Unlikely in system lifetime 1025 to 1026

Improbable Very unlikely to occur 1026 to 1027

Inconceivable Cannot believe that it could occur ,1027

Table 15.2 Risk matrix: categorization of severity of consequencesCategory Definition

Catastrophic Multiple loss of lifeCritical Loss of a single life

Marginal Major injuries to one or more persons

Negligible Minor injuries to one or more persons


Functional safety standardsFunctional safety standards nonexhaustively include provisions for:

• SIL determination

• Project management

• Architecture/redundancy

• Probability of failure on demand

• Equipment selection

• Software

• Proof testing

• Verification & validation

• Audits

• Assessments

• Management of change

• Competency

They are performance based rather than prescriptive; it is essentially a matter of

demonstrating fitness-for-purpose throughout the life of the installation. Although a

variety of certificates are offered (with varying credibility) by various parties for differ-

ent aspects of compliance there is no requirement within the standards for certification

of anything.

A key point to bear in mind is that the “SIL” is nominated for each individual

function, that is, the required effects to suppress the hazard associated with a given

cause. A high pressure or high temperature may cause the same valve(s) or drives(s) to

Table 15.3 Risk matrixConsequence

Key:

• Red: Class I—Unacceptable

• Orange: Class II—Undesirable

• Yellow: Class III—Tolerable

• Green: Class IV—Acceptable


trip; these would typically constitute two separate functions; one temperature, one

pressure. The trips may cause a variety of additional actions but it is only those that

are necessary for the suppression of the hazard that constitute the safety “function.”

Note also that protection functions are typically required to be implemented inde-

pendently of the control systems, the failure of which may give rise to a demand on

the protection function.

Safety integrity level/LOPALOPA is a HAZAN tool very commonly used in certain industries, which works to

quantify the risks associated with hazards identified in a HAZOP exercise in a more

sophisticated way than the Risk Matrix.

The approach is founded in standards to do with specification of the required

functional safety standards for electrical/electronic/programmable electronic safety-

related systems.

The functional safety standard IEC 61508 and its process sector specific derivative

IEC 61511 detail the approach to be employed throughout the design, implementa-

tion, operation, and maintenance of such systems.

Compliance with these standards is not mandatory, but they are held to represent

good practice. The performance requirements of the functions are tied to the risk

reduction factor (RRF) target identified for them and are allocated to one of four

“Safety Integrity Levels” (SILs):

SIL 1 RRF .10 to #100

SIL 2 RRF .100 to #1,000

SIL 3 RRF .1,000 to #10,000

SIL 4 RRF .10,000 to #100,000

In practice, SIL 1 and 2 are relatively straightforward to meet and will be substan-

tially achieved through the application of historical good practice for such functions.

SIL 3 is distinctly more challenging and will almost certainly require some redun-

dancy in the system provision, for example, 1 out of 2, 2 out of 3 voting.

SIL 4 is next to impossible to comply with. If a SIL 4 requirement is identified the

likelihood is that (a) your process design is seriously flawed; look to enhance inherent

safety and (b) your approach for identifying the risk reduction requirements (“SIL

determination”) is wrong or incorrectly calibrated.

Trips and Interlocks are known as Instrumented Protection Functions (IPFs) which

may be used to help reduce the risk associated with process hazards.

A SIL 1 requirement is routine; perhaps 10�20% of trip functions. SIL 3 is quite

exceptional; less than 1% of trip functions. Many IPF have a risk reduction require-

ment of less than a factor 10 and therefore are not SIL rated.

SIL determination may be undertaken by a variety of approaches, for example, risk

graph, risk matrix, layer of protection analysis, fault tree, which use more or less rigor in the


assessment of risk, the risk reduction contribution from other provisions (e.g., relief valves),

and the acceptable (tolerable) level of risk associated with the hazard to be protected against.

Although compliance with the SIL target is largely a matter for the instrument

discipline, the identification of the SIL target is very much a process concern.

There will typically be a range of instrument system design options and effective

management of these issues requires a dialogue between the instrument and process

disciplines.

This is potentially fraught territory; provisions within the standard are often

cited out of context or without due consideration of the particular circumstances. It is

all too easy for an “expert” to weave a plausible but wrongheaded argument. Absolute

compliance is something that we approach asymptotically; there typically comes

a point where the marginal gain in integrity does not warrant the additional expendi-

ture in resources.

FORMAL METHODS: SUSTAINABILITY

Sustainability is a highly politicized word, but we chemical engineers know exactly

what sustainability means, because the IChemE has helpfully told us not just what it

is, but how to quantify it, in their Sustainability Metrics. As engineers we understand

that if the company goes bust, our business plan was not sustainable, so there are

metrics in the IChemE document which measure this aspect of sustainability, as well

as some of the fluffier ones.

IChemE metricsTo quote the sustainability metrics document (see “Further Reading”):

The metrics are presented in the three groups3.1. Environmental indicators3.2. Economic indicators3.3. Social indicators

which reflect the three components of sustainable development.Not all the metrics we suggest will be applicable to every operating unit. For some units

other metrics will be more relevant and respondents should be prepared to devise and reporttheir own tailored metrics. Choosing relevant metrics is a task for the respondent. Nevertheless,to give a balanced view of sustainability performance, there must be key indicators in each ofthe three areas (environmental, economic, social).

Most products with which the process industries are concerned will pass through manyhands in the chain Resource extraction—transport—manufacture—distribution—sale—utili-zation—disposal—recycling—final disposal.

Suppliers, customers and contractors all contribute to this chain, so in reporting the metricsit is important that the respondent makes it clear where the boundaries have been drawn.

As with all benchmarking exercises, a company will receive most benefit from these dataif they are collected for a number of operating units, over a number of years, on a consistentbasis. This will give an indication of trends, and the effect of implementing policies.


A note on ratio indicatorsMost of the progress metrics are calculated in the form of appropriate ratios. Ratio indica-

tors can be chosen to provide a measure of impact independent of the scale of operation, orto weigh cost against benefit, and in some cases they can allow comparison between differentoperations. For example, in the environmental area, the unit of environmental impact per unitof product or service value is a good measure of eco-efficiency. The preferred unit of productor service value is the value added. . ., and this is the scaling factor generally used in thisreport. However, the value added can sometimes be difficult to estimate accurately, so surro-gate measures such as net sales, profit, or even mass of product may be used. Alternatively, ameasure of value might be the worth of the service provided, such as the value of personalmobility, the value of improved hygiene, health or comfort. But a well-founded and consistentmethod of estimating these ‘values’ must be presented

The metrics are calculated under the following headingsResource usage—Energy; Material (excluding fuel and water); Water; LandEmissions, effluents and waste—Atmospheric impacts; Aquatic impactsEconomic indicators—Profit, value and tax; InvestmentsSocial Indicators—Workplace; Society

So the engineer’s approach is (as ever) one of quantifying as best we can and then

balancing costs and benefits. We do not set the value of all environmental goods to

infinity, and the value of a company staying in business to zero.

SPECIFICATION OF EQUIPMENT WITH SAFETY IMPLICATIONS IN MIND

IntroductionThere is an excellent and concise treatment of the principles of safety in design in

“Process Plant Design and Operation,” which I do not propose to replicate in full

here, but there is a useful introductory statement and a few overarching principles:

The design should ensure a secure containment system. It must be robust and capable of han-dling both over and under-pressure condition plus temperature excursions where appropriate.

The design should avoid one event setting off a larger event. . ..If the process handles flamma-ble materials the sources of ignition must be kept to a minimum. It should be tolerant of small firesand designed to minimize the frequency of large fires and/or explosions In the case of corrosivefluids the design should be tolerant of corrosion both inside and outside the containment.

PrinciplesPersonal and process safetyMuch public discussion of health and safety issues (and many daytime TV adverts)

focus on personnel/personal safety issues like slips, trip, and falls and the like. In many

cases there is legislation which guides us as to how to design out these personal

hazards, though many (including IChemE President and HSE Chair, Judith Hackitt)

are now arguing that this kind of health and safety legislation is being commonly


misused by ambulance chasing lawyers and lazy public officials in a way which is

bringing it into disrepute.

“Process Safety,” however, tends to focus on the small subset of these risks which

have the potential for very serious incidents in industries handling large quantities of

hazardous materials.

Release of large quantities of toxic substances, major fires, and explosions are very

serious issues with the potential for multiple fatalities. These are the ones which are

usually the primary focus of process plant design safety exercises.

AccessA common fault of beginner’s designs is a lack of provision of safe permanent access

to equipment. This is normally done via platforms and walkways made of open mesh

decking, and vertical and inclined (“ship’s”) ladders, all usually made of galvanized

mild steel.

These items are also available in glass reinforced plastic for chemical resistance, as

well as aluminum for expensive shininess (but on the one occasion I had a client who

specified aluminum instead of galvanized mild steel, it was all stolen the night after

delivery).

General• Manways should be 0.5 m in diameter minimum, and placed facing gangways.

Provision should be made for winching a man out.

• Doors should be 0.6 m wide minimum.

• There should always be two escape routes for operators, especially at the top of tall

columns.

Horizontal access• Platforms should come with toeboards and 1 m high handrails. Platforms, walk-

ways and stairways should not be obstructed by pipes or equipment up to a height

of 2.25 m.

• Design loads on decking are specified in BS45492 as:

Light Duty (1 person) 3.0 kN/m2

General (regular two-way pedestrian traffic) 5.0 kN/m2

Heavy duty (high-density pedestrian traffic) 7.5 kN/m2

Vertical access• Minimum height between floors should generally be at least 3 m, and minimum

headroom under piperacks, cable tray, and so on not less than 2.25 m.

• Intermediate steps are required for elevation changes over 400 mm.


• Stairs are preferred over ladders for main vertical access, and ladders should be

hooped over 1.5 m, and ship’s ladders should usually be avoided. (Note that this

order of preference is, as is so often the case, in descending cost order.)

• Maximum ladder height without a landing: 7.5 m.

• Ladders should be arranged so that users face into equipment, not out into space,

and they should not be attached to the supports for hot pipes, to avoid distortion

by expansion forces.

• A clear 1 m square should be allowed on the plan layout for a ladder.

Flammable, toxic, and asphyxiant atmospheresExplosive atmospheres: DSEARThe Dangerous Substances and Explosive Atmospheres Regulations (DSEAR) require

risk assessment and, ideally, elimination of hazards associated with flammable and

explosive substances. The most important aspect of this legislation for the plant

designer is to do with classification of areas where explosive atmospheres may occur.

This has a major impact on both equipment specification and plant layout.

DSEAR and other directives and standards require that equipment and chambers

which may feasibly contain explosive atmospheres as a result of gases, vapors, mists, or

dusts be “zoned” based on the probability of occurrence of an explosive atmosphere.

The probability is usually assessed qualitatively, but for those who really like num-

bers, HSE gives approximate figures for zoning gas/vapor/mist hazards as follows:

• Zone 0: Explosive atmosphere for more than 1000 h/year

• Zone 1: Explosive atmosphere for more than 10, but less than 1000 h/year

• Zone 2: Explosive atmosphere for less than 10 h/year, but still sufficiently likely as

to require controls over ignition sources.

(The corresponding dust classifications are Zones 20, 21, and 22, respectively.)

Ignition sources have to be controlled within these zones. This may require the

exclusion of certain types of equipment, or the use of special “ATEX-rated” drives and

so on. (ATEX ratings code 1, 2, and 3 correspond to Zones 0, 1, and 2, respectively.)

If there is residual risk of explosion, consideration needs to be given to provision

of blast walls, safe paths for discharge of relief vents, explosion-hardened plant and

control buildings, and design of tanks and other equipment to withstand explosion.

We tend, when designing plant, to need numbers to work with in order to quan-

tify risks. In the case of flammable and toxic hazards, we have the upper and lower

flammable/explosive limits for a material (and its flash point) and a range of workplace

exposure limits for acute and chronic exposure to toxic substances defined as follows:

Flammability hazards• Lower Explosive Limit, LEL/LFL—The minimum concentration of vapor in air

below which the propagation of flame will not occur in the presence of an


ignition source. Also referred to as the lower flammable limit or the lower explo-

sion limit.

• Upper Explosive Limit, UEL/UFL—The maximum concentration of vapor in air

above which the propagation of flame will not occur in the presence of an ignition

source. Also referred to as the upper flammable limit or the upper explosion limit.

• Flashpoint: The minimum temperature at which a liquid, under specific test con-

ditions, gives off sufficient flammable vapor to ignite momentarily on the applica-

tion of an ignition source.

• Flammable liquids are classed based on flashpoint as:

• Extremely flammable—Liquids which have a flashpoint lower than 0�C and a

boiling point (or, in the case of a boiling range, the initial boiling point) lower

than or equal to 35�C.• Highly flammable—Liquids which have a flashpoint below 21�C but which are

not extremely flammable.

• Flammable—Liquids which have a flashpoint equal to or greater than 21�Cand less than or equal to 55�C and which support combustion when tested in

the prescribed manner at 55�C.• Inflammable—confusingly for nonnative speakers, inflammable means the same

thing as flammable (or perhaps even extremely flammable). Its use should

therefore be avoided.

• The higher up this list a substance is, the more we should seek to substitute it with

something less flammable (or failing that the more precautions we would have to

take).

Toxic hazardsThe long-term exposure limit (LTEL) is the time-weighted average concentration of

a substance over an 8-h period thought not to be injurious to health.

The short-term exposure limit (STEL) is the time-weighted average concentration

of a substance over a 15 min period thought not to be injurious to health.

The HSE Publication EH40 gives exposure limits for a wide range of chemicals

(see “Further Reading”).

If we identify excessive exposure to toxic chemicals in our design, we should first

consider substituting the materials which produce toxic hazards. Failing that, we can

use engineering controls such as ventilation, avoidance of enclosure, controlling access

to contaminated areas, and so on.

Note that there are many substances which are both toxic and flammable, and

both hazards should be considered simultaneously.

There may be some substantially enclosed areas which may have flammable, toxic

or asphyxiating atmospheres which we cannot design out. These are classified as con-

fined spaces, and access to them has to be tightly controlled.


Confined space entryAccording to the HSE, “A confined space is a place which is substantially enclosed (though

not always entirely), and where serious injury can occur from hazardous substances or conditions

within the space or nearby (e.g. lack of oxygen).”

It is most often the case that the relevant hazard is the possibility of presence of

asphyxiating, flammable, or toxic gases, or the absence of oxygen, so there is some

overlap with explosive area zoning.

Entering such spaces (even quite shallow trenches can qualify, as an operator can

bend down and place his head in the hazardous atmosphere) has the potential for

multiple fatalities, and formal risk assessment is required by law before any entry.

This will often require any operators entering such a space to have special training

and equipment. Entering confined spaces (even if supposedly only for a moment) is

a big deal. It is a bad idea to have any equipment requiring operator access in a con-

fined space.

If this cannot be designed out, the safest kind of confined space is one with a

direct drop straight from the surface to the working area. Having to navigate turns

and level changes in a confined space is a very risky operation—few except mine res-

cue teams have the necessary skills and equipment.

Lockable covers on confined spaces are a good idea, and it might be best not to

have an internal access ladder. Here is an example of an undesirable layout I came

across (Figure 15.2):

In this example, properly trained staff could have been winched in, and the

absence of a ladder would dissuade untrained staff from just popping down to look at

Figure 15.2 Example of poor layout, including hanging cables and nonrecommended accessladder.


something in a way which has led to many deaths in the past. Multiple fatalities have

occurred on many occasions in which untrained operators have gone to the rescue of

others who went before them, and are overcome by the same conditions. In Qatar, in

2012, seven expatriate operators were killed in a single incident of this nature on the

first day I worked there, and there have since been multiple fatality incidents in the

United Kingdom.

Wet/dusty atmospheres: ingress protection (IP) ratingsEquipment needs to be specified so that it is suitable for its environment with respect

to particle and water ingress. The most commonly used standard is the IPXX stan-

dard, where the first X represents a solid particle ingress standard, and the second X a

water tightness standard. 0 is no protection, and 5 is dust protection in the case of the

solids standard, 8 is immersion below 1 m in the case of the water standard.

Submersible equipment needs to be rated at IP68 or better, and control panels at

IP55 or more. Indoor equipment may be rated as low as IP22 (the standard for

domestic power sockets), protected only from fingers and water drips over a short

period.

SPECIFICATION OF SAFETY DEVICES

No safety device is 100% reliable, so the use of safety devices is only indicated where

it has not been possible to design out hazards, which is always the preferable option.

TYPES OF SAFETY DEVICE

Overpressure protectionInexperienced engineers have a tendency to do one of two things, either they put

pressure relief valves (PRVs) everywhere or (much worse) do not include them where

they are needed.

The first error adds significant cost to both capex and opex and possibly produces

a net decrease in safety through increased complexity. The second error can be a disas-

ter waiting to happen.

I do not intend to go into the detail of relief valve sizing, about which a whole

book can be written; instead I will cover some of the basic scenarios in which a PRV

could be required.

The real skill of sizing relief valves is not in grinding through the standard sizing

calculations, it is the application of engineering judgment to identify the scenarios in

which over pressurization could occur, and determining the reasonable worst-case

relief load in such an event.


This section is written with reference to API520 and API521, the Oil & Gas

Industry standard for PRV sizing (see “Further Reading”). In the United Kingdom/

Europe, the Pressure Equipment Directive will need to be complied with in all indus-

tries, but the API standards contain some useful rules of thumb for PRV application

which have no equivalent in UK or European standards. API521 discusses some com-

mon scenarios in which an overpressure (and therefore a “relief case”) may occur.

The most common of these are as follows:

Closed outlets (on vessels)In the event that all the outlets from a vessel are closed off (perhaps due to manual

valves being closed through an operator error or automatic valves failing), system

overpressure may occur. While this is quite unlikely, as engineers we have to assume

that all the outlets to a particular vessel might be closed if it is physically possible.

The key issue is to determine if the highest achievable pressure in the vessel is

above the design pressure. In many cases this may mean comparing the maximum-

rated pressure of upstream pumps or supply vessels to the vessel in question.

Inherently safe design implies that design pressures throughout the entire system

which might be over pressurized by such an event are consistent. For example, the

rated pressure of the upstream pumps will be equal to the pressure of the downstream

vessels/valve, etc.

Burst tube caseHeat exchangers will almost always contain fluid at higher pressure on one side of the

tubes than the other, so a burst tube will result in the high pressure fluid leaking into

the low pressure side (including, in some cases, flashing of the fluid) which might ulti-

mately cause a catastrophic failure.

In this instance it must be noted that we are not protecting against cross-contamination,

but protecting the exchanger/pipework against catastrophic failure and consequent loss of

containment.

We can “dismiss this safety/relief case” (i.e., create an inherently safe design) if the

test pressure on the low pressure side is higher than the design pressure on the high

pressure side.

Cooling water/medium failureCooling can be used deliberately to create a pressure drop within a system. In these

cases a loss of the cooling medium may lead to increased pressure (a similar scenario

can occur through loss of reflux cooling).

An inherently safe design will design vessels for the maximum pressure achievable

in the event cooling is lost.


Blocked in (hydraulic expansion)This scenario often occurs with liquids in heat exchangers in two situations:

1. A cold liquid side of a heat exchanger becomes blocked in while the hot side con-

tinues to flow. Depending on the temperature difference, increased heating may

cause expansion or vaporization leading to overpressure.

2. Liquid in a line may be blocked in (e.g., by an operator closing a valve in error). If the

liquid is normally below ambient temperature (or it has trace heating) it may expand

on heating and cause overpressure. While the expansion may be small, in the case of

incompressible fluids the pressure can quickly increase and cause a problem.

Exterior fire caseIn the event that a fire occurs immediately outside a vessel, the contents will be heated

and can over-pressurize the vessel. This is a very difficult case to dismiss, however it

can be dismissed if the vessel is at least 7.6 m above the base of the fire, if the vessel is

protected by fire-resistant insulation, or in some other way.

Pressure relief valvesPRVs (Figure 15.3) are spring loaded valves which open automatically at a set pressure,

releasing the contents of a pipe or vessel to atmosphere or to a vessel depending on

design detail. While in theory they should not, in practice PRVs tend to leak increas-

ingly over time so they are not the best choice where complete containment is crucial.

Figure 15.3 Pressure relief valve.


Bursting discsBursting discs (Figure 15.4) are an engineered metal plate fitted across a pipe which

bursts at a specified pressure, allowing the pipe contents to pass to atmosphere or ves-

sel. They are a better choice than PRVs where containment is crucial, but once burst,

they need to be replaced. They are quite often specified as protection upstream of a

PRV to prevent fugitive emissions.

Blowout panels, etc.Typical gas/air or dust/air explosion overpressures are of the order of 10 bar. It may

not be practical to design vessels to withstand the overpressure (Figure 15.5).

Figure 15.4 Bursting disc. Copyright image of the Safe-Gard Bursting Disc reproduced courtesy of Elfab.

Figure 15.5 Blowout panel. Copyright image reproduced courtesy of Elfab.


Instead, in a similar manner to bursting discs, the roof or wall of a building or ves-

sel can be engineered to fail first, diverting a blast in a safe direction, and minimizing

damage within the protected space.

It is common practice for the roof of fixed roof atmospheric storage tanks to have

deliberately weak seams for this purpose.

Under-pressure protectionVacuum relief valveThe lids of large tanks such as those used for storage of products and intermediates on

oil and gas facilities may only be designed to withstand pressure, and may be readily

imploded by surprisingly small degrees of vacuum. They are therefore usually protected

by vacuum relief valves (or combined “vent/vac” or “relief/vac” valves) (Figure 15.6).

Figure 15.6 Vacuum relief valve.


Static protectionNonconducting fluids such as hydrocarbons flowing rapidly through pipes or strongly

agitated in vessels can produce sufficient static electricity to self-ignite by spark dis-

charge. Contrary to popular belief, metal pipes are actually more likely to exhibit

such charging behaviors than nonconductive materials (Figure 15.7). Though the risk

may be reduced by reducing liquid velocities, such an approach is unlikely to be reli-

able or cost-effective enough for complete elimination of risk. Such systems need to

be safely earthed to prevent fires caused in this way.

Gas detectorsWhere toxic or flammable gases may be present, permanent gas detection and alarm

systems may be required to ensure personnel and plant safety (Figure 15.8).

Figure 15.7 Static protection measure. Copyright image reproduced courtesy of Newson Gale.


Emergency shutdown valvesWhere it is desired to cut off flow of a component quickly in a potentially hazardous

circumstance, valves may be installed which reliably shut off flow in that condition

(Figure 15.9). These are known as shutdown valves (SDVs) or emergency shutdown

valves (ESVs). They are common features in the oil and gas industry and other safety

critical industries.

Figure 15.8 Gas detector heads. Copyright image reproduced courtesy of Crowcon.

Figure 15.9 Emergency shutdown valve. Copyright image reproduced courtesy of Ascendant.


They are actuated valves, which introduces a number of risks to reliability. The

hazardous condition has to be detected, a signal has to pass to the valve, and the valve

actuator has to work. All of these things may need to happen very reliably in a condi-

tion where the plant is on fire, and main plant power is offline.

For this reason, ESV actuators are normally of the spring return type or actuated

by fail-safe fluid power systems, and any signals wiring is fireproofed.

Flare stacksWhen PRVs are lifted by overpressure on a gas processing facility, it is undesirable to

vent large quantities of flammable gas to atmosphere. Burning the gas in a flare stack

makes it safe (Figure 15.10).

Figure 15.10 Flare stack at the Shell Haven Refinery, UK.


Flare stacks are also used to handle gas produced during maintenance and repair

activities, plant bypasses, and so on, as well as gas which is considered economically

nonviable to recover.

ScrubbersAn alternative way of removing dangerous (usually toxic, nonflammable) substances

from a vented stream is through the use of emergency scrubbers (Figure 15.11).

Figure 15.11 Scrubber.


Water spraysFixed water spray cooling systems are commonly provided on the tanks used to

store flammable hydrocarbons in petrochemical facilities (Figure 15.12). There is

a commonly used standard, “NFPA 15: Standard for Water Spray Fixed Systems

for Fire Protection,” but the design of such safety critical systems is best left to

specialists.

Quench tanksWe can arrange for the contents of a vessel containing a reaction which might run

away to be dumped quickly to a tank whose physical or chemical conditions stop the

reaction very quickly.

Figure 15.12 Water spray system in operation. Copyright image reproduced courtesy of Lechler.


FURTHER READINGAnon, 2008. Guidelines for Hazard Evaluation Procedures Center for Chemical Process Safety.Anon, 2008. Inherently Safer Chemical Processes: A Life Cycle Approach Center for Chemical Process Safety.Anon, 2011. EH40/2005 Workplace Exposure Limits Containing the List of Workplace Exposure Limits

for Use with the Control of Substances Hazardous to Health Regulations 2002 (as amended). Healthand Safety Executive.

Anon, 2012. Guidelines for Engineering Design for Process Safety Center for Chemical Process Safety.API STD 520-1, 2014. Sizing, Selection, and Installation of Pressure-Relieving Devices in Refineries:

Part I—Sizing and Selection (9th ed.). American Petroleum Institute.API STD 521, 2014. Pressure-Relieving and Depressuring Systems (6th ed.). American Petroleum Institute.Azapagic, A., 2002. Sustainable Development Progress Metrics Recommended for Use in the Process

Industries. Institution of Chemical Engineers, London.Scott, D., Crawley, F., 1992. Process Plant Design and Operation: Guidance to Safe Practice. Institution

of Chemical Engineers, London.

SOURCESFigure 15.6: Free image reproduced from Solids Wiki, http://solidswiki.com/index.php?title5

Vacuum_Relief_Valves.Figure 15.10: Image reproduced under Creative Commons License (http://creativecommons.org/

licenses/by-sa/3.0/). Taken from http://en.wikipedia.org/wiki/Gas_flare#mediaviewer/File:Shell_haven_flare.jpg.

Figure 15.11: Image reproduced under Creative Commons License (http://creativecommons.org/licenses/by-sa/3.0/). Taken from http://upload.wikimedia.org/wikipedia/commons/8/87/G_G_Allen_Steam_Plant%2C_scrubber.JPG.






http://solidswiki.com/index.php?title=Vacuum_Relief_Valves




http://creativecommons.org/licenses/by-sa/3.0/


http://en.wikipedia.org/wiki/Gas_flare%23mediaviewer/File:Shell_haven_flare.jpg

http://en.wikipedia.org/wiki/Gas_flare%23mediaviewer/File:Shell_haven_flare.jpg



http://upload.wikimedia.org/wikipedia/commons/8/87/G_G_Allen_Steam_Plant%2C_scrubber.JPG

http://upload.wikimedia.org/wikipedia/commons/8/87/G_G_Allen_Steam_Plant%2C_scrubber.JPG

PART 5

Advanced DesignDesign optimization proceeds iteratively by stages in professional practice, with an

increasing degree of multidisciplinarity, checking, attention to detail, and quality con-

trol. This process is quite distinct from academic process integration or optimization

techniques.

CHAPTER 16

Professional Practice

INTRODUCTION

Engineering is a collaborative human activity. Humans vary in their technical, intel-

lectual, social, and verbal capabilities, though engineers may not encompass the full

range of variation.

While there may be a few individuals with high levels of ability across the range,

good collaboration allows those with high technical ability but poor social or verbal

abilities (you know, nerds) to complement those with less technical ability, but more

charisma and communication skills (the managers of the future). The most creative

designers may be rather questioning of authority and intolerant of rules, for the same

reasons as they are good at finding creative solutions and are consequently often

freelancers.

There are a number of formal interactions between engineers which facilitate

communication with those who may be uncomfortable with unstructured conversa-

tions. Those which count as work might broadly be termed design reviews, negotia-

tions, and formal procedures, though there are crossovers between these categories.

There is another type of interaction which is very useful to engineers, follows a

well understood format, but doesn’t usually count as work: discussing what not to do

based on personal anecdotes.

Harvey Dearden has produced a little book which helpfully sets out quite a lot of

the unwritten rules of professional engineering culture, and which I would recom-

mend to all new engineers (see “Further Reading”)—though you can skip the Jane

Austen chapter.

GENERAL DESIGN METHODOLOGY

• Design

• Review

• Negotiate

• Revise

• Redesign

• Repeat until out of resources


INFORMAL DESIGN REVIEWS

Consultation with equipment suppliersThe people who sell unit operations and other process kit usually have a very deep

knowledge of its practical characteristics, and those of competing products. Obviously

they would like to sell you their kit, but they will scarcely ever lie in order to achieve

this. See a good few of them, and you can learn how to play a game which will allow

you to incorporate their detailed practical knowledge into your designs.

Consultation with electrical/software partnersSometimes you will have in-house electrical or software engineers, but nowadays

there will normally be an external electrical installer, motor control center (MCC)

supplier and software designer. These may all be under one roof, or there may be

combinations. If you don’t have in-house specialists to back you up, combinations

are better, but as with all in engineering, the less you know the more you pay.

Electrical and software components of the job are very significant, and are perhaps

the single biggest opportunity for cost overruns at installation and commissioning

stage, so there is a potential liability to manage. There are also big opportunities for

cost savings if a well-integrated and controlled design can be devised.

Consultation with civils/buildings partnersAs with electrics, civils and buildings are often not designed in-house. Civil engineer-

ing companies often work on very small margins, and may consequently have a some-

what inflexible approach to contract documentation.

They are far more likely to employ quantity surveyors (QSs) than other disciplines.

QSs are a kind of engineering accountant-cum-lawyer, and are not well loved by

engineers. They are characterized in an old joke as the “people who go in after the

war is lost and bayonet the wounded.”

These companies are also much more likely to sue partners if things do not go

well in construction than other disciplines. Experienced engineers are consequently

generally very cautious in their dealings with civil partners, though design and costing

are normally separate parts of the operation for civil contractors and consultants.

There is, however, potential for both good savings and, more importantly, good

control of potential construction stage cost overruns, if the civil aspects of design are

well integrated and defined.

The things you learn in these discussions can also alter the starting point of your

future designs in such a way as to give them better cross-discipline integration.


Consultation with peers/more senior engineersSome people like to keep things to themselves, and some need a sounding board to

develop ideas. I am in the second category, so I have learned a lot from others during

these interactions. The other party does not, however, always have to be a more expe-

rienced engineer. Sometimes you just need to get an idea out there and play with it

to see its strengths and weaknesses. Sometimes it needs a fresh pair of eyes to see

things which an idea’s author cannot.

Unless you are in the fortunate position of being allocated a personal mentor,

senior engineers may often not have a great deal of time to talk to you, but few will

refuse to help you out with a knotty problem.

Peers and near-peers will probably be the people you spend most time discussing

things with and, while they can be useful, you should bear in mind that they are

more likely to suggest investigating blind alleys, or using inappropriate design techni-

ques than old hands.

FORMAL DESIGN REVIEWS

Interdisciplinary design reviewThe point of design reviews is to make sure that the design is reasonably optimal in

the opinion of more senior engineers.

Designs need to balance the needs of (at a minimum) the process, mechanical,

civil, and electrical engineering disciplines. Considering installation and commission-

ing issues is also mandatory.

The design review will therefore bring together the senior engineers of each

discipline within the company, and will focus on the design drawings.

The atmosphere of such meetings is normally reasonably friendly, but engineers

can be quite challenging. There may also be internal company political issues at play.

Strong chairmanship and negotiation skills are a requirement if such meetings are to

work well.

Beginners will learn a lot in such meetings, and should miss no opportunity to

attend one, even though the first few for their own designs might include some learn-

ing experiences they might not entirely enjoy.

Value engineering reviewValue engineering reviews have many similar characteristics to the design reviews of

the last section, though their focus on cost and value will attract more management

types, sales and marketing people, and so on.

251Professional Practice

As the name suggests, they are an attempt to get the price right—usually aiming

for a downward adjustment.

These are sometimes conducted in the presence of client representatives, which

can on occasion result in a rethink from scratch of some constraint on the design

which the client had not realized the full implications of.

Safety engineering reviewThis is most often a formal approach such as a HAZASS or HAZOP, but it is cultur-

ally very similar to the last two types of review.

QUALITY ASSURANCE AND DOCUMENT CONTROL

Engineers . . . are not superhuman. They make mistakes in their assumptions, in their calcula-tions, in their conclusions. That they make mistakes is forgivable; that they catch them isimperative. Thus it is the essence of modern engineering not only to be able to check one’sown work but also to have one’s work checked and to be able to check the work of others.

Henry Petroski

Control of the design process and design documentation is incredibly important in

professional practice. There are strong negative safety implications of poor control of

the design process.

Engineering documents are, for example, always marked with revision numbers

and dates. If the document is changed in any way, it is given a new dated revision

number. This prevents a lot of potential confusion.

Once a design is at the point where the major design difficulties have been

resolved (after usually 2�3 revisions), revision zero will normally be issued. Any

remaining issues on the P&ID may be highlighted within an irregular outline and

marked HOLD. Once this is done, the design is considered sufficiently complete for

the drawings to be used as the basis for commencement of design and procurement.

Once it has been decided that the design is sufficiently complete to issue for con-

struction, the design will be frozen, which is to say that no significant change will be

allowed.

So document change control is very important—it is usual to specify the engineer-

ing discipline and degree of seniority required to change given documentation, and

for there to be checking of all changes by a second engineer of an appropriate disci-

pline with a specified level of seniority.

The ISO 9000 series European Standard (derived from BS5750, a British Standard

for quality assurance in produce design and manufacture) gives a very widely used for-

mal methodology for the control and audit of all processes. The ISO 9000 series is

now applied away from product design and manufacturing, and there are less formal


approaches used everywhere in engineering which cover more or less the same

ground.

ISO 9000 requires that you have documented procedures for:

• Control of Documents

• Control of Records

• Internal Audits

• Control of Nonconforming Product

• Corrective Action

• Preventive Action

It also requires that you keep permanent records of the following:

• Management reviews

• Education, training, skills, and experience

• Evidence that processes and product or service meet requirements

• Review of customer requirements and any related actions

• Design and development including: inputs, reviews, verification, validation, and

changes

• Results of supplier evaluations

• Traceability where it is an industry requirement

• Notification to customer of damaged or lost property

• Calibration

• Internal audit

• Product testing results

• Nonconforming product and actions taken

• Corrective action

• Preventive action

These are records you need to provide evidence of following your processes.

Professional process plant design is a very highly controlled and documented pro-

cess, and your design decisions are recorded for future reference. It should be borne

in mind that your decisions may even have to be defended in court long in the future.

Like many other engineers, I keep my own dated handwritten notebooks in which

I record my decisions. They can come in handy if you ever have to appear in a court

case in respect of a decision you made 10 years previously. Contemporaneous notes

are admissible evidence, as well as an aide-memoire.

INFORMAL DATA EXCHANGE

Engineers love to talk about things which have gone wrong. This might seem like

gossip, and there may be a component of that, but actually it spreads the knowledge

of what doesn’t work.

253Professional Practice

Engineering experience consists far more of knowing what doesn’t work than

knowing what does.

FURTHER READINGAnon, 2008. ISO 9001:2008 Quality Management systems—Requirements. International Standards

Organization, Geneva.Dearden, H.T., 2013. Professional Engineering Practice: Reflections on the Role of the Professional

Engineer. Createspace.






CHAPTER 17

Beginner’s Errors to AvoidINTRODUCTION

It takes an engineer to undertake the training of an engineer and not, as often happens, atheoretical engineer who is clever on a blackboard with mathematical formulae but useless asfar as production is concerned.

The Rev E.B. Evans

Academic myopiaMany of the errors which follow are often hard-wired into academic design techni-

ques. In summary, best simply forget everything you were told in academia about pro-

cess design. Those who taught you have almost certainly never designed a unit

operation which has been built, let alone a whole plant.

Lack of consideration of needs of other disciplinesReal process plant designers have to take into consideration the needs and desires of

several other engineering disciplines, most notably civil, mechanical, electrical, and

software in that order. The idea that a chemical engineer can sit down and design a

plant in glorious isolation comes only from the inadequacy of links between disci-

plines and professional practice in academia.

Lack of consideration of natural stages of designIt is one thing to consciously accelerate a program by rolling a couple of stages of

design together. It is quite another to attempt to apply techniques intended for a par-

allel universe where these stages do not exist.

Excessive noveltyAcademics progress in their careers by being radically innovative. Being novel is more

important than being right to researchers who wish to be published, and many teach

their students to value novelty too. Professional engineers are no more novel than

absolutely necessary. Being right is far more important to us than being original.

Lack of attention to detail“Block flow diagrams” are commonplace in university, and process flow diagrams

(PFDs) are commonly the highest level of definition of plant interconnectedness. I

have never used a block flow diagram in professional practice—I often go straight to


piping and instrumentation diagrams (P&IDs). I only do PFDs if I need them to

envisage mass flows, or a client asks for them.

This is symptomatic of the different levels of attention applied by professionals and

theorists. Having taught process design to many university lecturers, I know that it is

commonplace for the mediocre ones to think that all design problems below the level

of mathematical theory are trivial. Only the exceptional ones are willing to throw

themselves in to the point where they learn that the devil is in the detail.

There is a useful checklist in “Practical Process Engineering” to check for P&ID

completeness which will draw a beginner’s attention to frequently neglected issues,

which I have reproduced in Appendix 4.

Lack of consideration of design envelopeUniversities are graduating students who have never considered anything beyond static

steady state design. Even Master’s level “Advanced Chemical Engineering” modules use

the simplified model so that they can spend longer on pinch analysis. This model is at

best a simplified one used for the newest beginners—this is not how design is done.

The design envelope considers all relevant aspects of a specific proposed site in

determining which approach is likely to be best. The regulatory environment, climate,

price of land, skills of available operators and construction companies, reliability of

power supply, risk of natural disasters, and proximity of people are often at least as

important as the theoretical yield of a process chemistry.

There is no right answer to design. The right design for a less developed country

will not be that for a more developed country. The right design for a client with a lot

of experience with a particular process will differ from that for another client.

Lack of consideration of construction, commissioning, and nonsteadystate operationThis is a subset of the above error. If your plant doesn’t work during commissioning and

maintenance it doesn’t work at all. Get it right—consider all stages of the plant’s life.

Principal maintenance activities to consider during design are isolation, release of

pressure, draining, and making safe by purge or ventilation. Remember to allow for

isolation of utilities as well. Purge lines should ideally be temporary to prevent back-

flow contaminating the reservoir.

Isolation, block, double-block, and double-block-and-bleed valves are used for

these duties, supplemented by spades, slip plates, and blinds.

If there is going to be hot maintenance (while the rest of the plant is working),

the layout must be suitable for this. Isolation of vessels must not isolate them from the

pressure relief system, though the possibility of connection to process via this route

needs to be considered.


Parallel and series installationBeginners seem to have little feel for the differences between parallel and series dupli-

cate installations, and when each is appropriate. They may, for example, think that the

headlosses of units in parallel are additive (they are not). The heuristic is: pumps in

series—add heads, pumps in parallel—add flows.

Lack of redundancy for key plant itemsStandby capacity can be something of a mystery to beginners. Generally speaking, if

an instrument or item of rotating machinery is crucial to plant operation, you need at

least one full size standby unit. For economic or other practical reasons you may alter-

natively choose to have three 50% duty units instead of two 100% units (duty/assist/

standby versus duty/standby).

If the item is really crucial, you might want to make sure that a common cause

failure of the units cannot happen. So you might specify, at a minimum, separate

cabling all the way back to the motor control center (MCC) for electrically driven

items, or follow the example of the oil and gas industry in having steam-powered

backups for crucial electrically driven pumps.

It is not only unit operations which may need backup; utility failure can lead to

hazardous situations. We need to assess the required reliability and include standby as

required early on. Electrical power to crucial items usually requires twin feeds and/or

generator/battery backup. Note that generator/battery reliability and the yielded

length of trouble-free operation costs money, and that generator supplied power is not

necessarily as “clean” as mains power. It may be that “load shedding” needs to be

specified, such that only the most essential plant processes have power backup.

Emergency cooling or reactor dumping to quench tank may be used to bring the

plant to a safe and reliable stop rather than provide for continued operation in the

event of utility failure.

Lack of consideration of processes away from core process streamAssume nothing. If the client has not told you that water, air, electricity, effluent treat-

ment, odor control, and so on are available free of charge and suitably rated for your

process, assume you need to provide them. If the client has told you they are, check

that you are happy with what is offered. Mark your P&ID with termination points

making clear where your scope begins and ends.

Storage often takes up more space, and presents more hazards than the main pro-

cess. Note that a few big tanks are probably cheaper than lots of small ones but the

consequences of failure are greater.

Designers need to consider emergency releases from the plant, allowing fenced off

out of bounds sterile areas for flares and vents, and adequate scrubbers for toxics.

257Beginner’s Errors to Avoid

We need to make sure drainage systems are adequately designed. We need to avoid

pits which might collect heavier-than-air flammable vapors if leaks could produce

them, unless these are specifically designed impounding basins to allow leaking flam-

mable vapors to burn off without damaging other equipment.

Tanks should be contained in bunds with 110% of the largest tank volume being

the usual minimum allowance. We need to consider precipitation/firewater drainage

requirements when designing bunds. They may need covering or additional capacity.

Access (which does not breach the bund) to any equipment inside the bund also needs

to be provided.

Lack of consideration of price implications of choicesThe best university level pricing techniques are greatly inferior to professional prac-

tice, and many things considered perfectly acceptable in academia are considered woe-

fully inadequate in professional practice. Making choices between technologies or

configurations without pricing them as well as you possibly can at that stage of design

is unprofessional. Don’t do it.

Academic “HAZOP”There is a thing called HAZOP (or sometimes CHAZOP) by many in academia

which consists of reviewing a PFD using a version of the HAZOP procedure to

generate the required control loops. This is neither HAZOP nor CHAZOP, and

this is not how we determine how to instrument and control our plants. If you

don’t know how to do the basic control of your plant, look at Chapter 13, and/or

ask an experienced engineer. You’ll only be doing it their way come the design

review anyway.

Uncritical use of online resourcesThe internet is full of all kinds of potentially useful resources. I once asked one of my

students the source of some pricing data, and he said “Chinese Websites.” Without

getting as obsessed with proper referencing as my research colleagues, we do need to

put a little more thought into the reliability of any internet resources we use.

There are all kinds of online calculators for sizing pipework, equipment, and so

on. There are sites which advertise chemicals and equipment for sale. Some of these

are good (such as, e.g., lmnoeng for fluid flow calculations), and some are very

misleading.

Professional engineers do not offer printouts of stuff from the internet as a substi-

tute for their own calculations based in reliable information. I have, however, been

known to use lmnoeng and the like, as a quick check that I have any novel hydraulic

calculations about right if there isn’t a second engineer available to check me. I don’t


assume I have it wrong if the website disagrees, but if it agrees with me, I am happy

to assume my calculations are about right.

Professional judgment is what engineers get paid for. Don’t do anything without

exercising it.

LACK OF EQUIPMENT KNOWLEDGE

Lack of knowledge of pump types and characteristicsThere are two main kinds of pumps (rotodynamic and positive displacement), whose

characteristics were explained in an earlier chapter. There are many subtypes of pumps

which differ from each other in nontrivial ways.

The type of pump selected affects the way the pump needs to be controlled and

protected, the precision of pumping, the suitability for a given fluid in terms of its vis-

cosity and solids content, the power utilization for a given duty, the maintenance

requirements, and so on.

Until beginner designers apply the knowledge of pump characteristics outlined in

the Tables 10.3 and 10.4, they will consistently make schoolboy errors in the selection

of pumps and surrounding systems.

Attempting to control positive displacement pump output with a valveThis is one of the knock-on effects of the broad class of errors covered in the last sec-

tion, and one of the commonest errors of those with little design experience (which I

have seen in supposedly bright young engineers applying for chartership).

Do not attempt to control the output of a positive displacement pump with an in-

line throttling valve—this does not work and will damage the pump.

Multiple pumps per lineThere is nothing at all wrong with having multiple pumps in parallel as an assist or

standby arrangement, but pumps in series are usually an error. Pumps in series, espe-

cially multiple positive displacement pumps in series, are not a feature of professional

designs.

Professionals know that we can get multiple stages of centrifugal pump in a single

unit if we want higher delivery head, and if you can’t get a pump to do your duty,

you are probably looking at the wrong kind of pump.

We also know that multiple positive displacement pumps in series do not work, as

we are throttling suction or delivery when they pump out of synch.

Lack of knowledge of valve types and characteristicsThere are essentially three broad classes of valve duties, as outlined in the earlier sec-

tion: isolating, on/off, and modulating control valves. Different industries use different


valve types for these duties, but all industries have these requirements. All designs

should reflect this understanding. Table 10.5 is intended to help beginners to under-

stand more about what is available. Actuated valves should be considered as rotating

machinery—if crucial to the process, standby capacity is required.

Lack of knowledge of actuator typesAs outlined previously, there are three broad kinds of actuators. Note that pneumatic

actuators require compressed air and additional control equipment (solenoid valves to

control airlines) to function.

Throttled suctionsDon’t try to control the output of a pump by throttling the suction, so as to avoid

cavitation, among other things.

Use of actuated bypass valvesBack when I was in university it was common to control the output of a positive dis-

placement pump with an actuated valve in a bypass to suction. It does at least sort of

work, but times have moved on, and we use inverters now. I never used this tech-

nique to control centrifugal pumps even back then.

Use of control valvesI personally don’t make a lot of use of in-line control valves for liquids nowadays at

all. While this is personal preference, the greater power efficiency of inverters is a fact.

In the United Kingdom there are tax breaks for using inverters because of this.

Multiple valves per lineIn university, they might have taught you a clever way of using multiple valves in the

same line with different lags to control flow based on multiple variables, but I would

recommend that you don’t try it in practice unless there is absolutely no other way of

achieving your aim. KISS. Try to control flow in a line only once. In any case most of

the multiple valves per line I see in beginner’s designs are not sophisticated cascade

control, they are simply errors.

Lack of tank drains and vents/other valves necessary for commissioningA word of warning—don’t upset the commissioning engineer. Commissioning engi-

neers want to be able to drain tanks down in a reasonable time—say 30 min. Make it

so. Air will need to come in to replace the fluid—be sure to include a vac/vent or

other valve to allow this.


Think about the commissioning operation—additional valves may be needed to

commission unit operations in isolation, or add services needed during commission-

ing. Put them in.

If you are unsure about what commissioning engineers need, ask them. If you do,

exercise professional judgment and be prepared not to add absolutely everything they

ask for. They are not employed to care about whether the company gets the job.

Lack of consideration of details of drainage systemsBadly designed drainage systems can be the cause of very serious problems—they can

allow the build-up of hazardous material from leaking equipment though undersizing

or lack of provision for removal of solids build-up, allow incompatible materials to

mix, carry toxic gases, fire or explosions from one section of the plant to another.

They are nontrivial.

Lack of sample pointsCommissioning engineers will also berate you for omitting the valves they need to

take samples while commissioning. As with all the things which upset commissioning

engineers, operating staff won’t thank you either. Think about where you will need

to take a sample to test whether a unit operation is working. Put a sampling valve in

there, or a more complex arrangement if containment is an issue.

Lack of isolation valvesEvery unit operation needs at least one isolation valve on every inlet and outlet.

Include it.

Lack of safety valvesNonreturn valves, pressure relief valves, pressure sustaining valves, etc.: if you haven’t

included them you haven’t really considered all that can happen on the plant. More

experienced engineers will hopefully add what you have omitted, but why not save

them the trouble?

Lack of redundancy for key valvesKey actuated valves may well need actuated standby valves, and all actuated valves on

units which are not themselves entirely duplicated are likely to need a bypass with iso-

lation and a manual standby control valve for maintenance in service.

LACK OF KNOWLEDGE OF MANY TYPES OF UNIT OPERATIONS

The law of the hammer (if all you have is a hammer, everything looks like a nail)

operates if you know too little about your options.


Universities seem to concentrate on a small selection of unit operations important

in the petrochemical industry. All chemical engineering students tend to see scrub-

bing, stripping, distillation, and drying several times during their course. The other

99% of unit operations are a mystery to them.

This book attempts in some of the tables in Chapters 10 and 11 to address the

issue of lack of knowledge of separation processes and so on. More generally, new

designers need to discuss the things they are doing with more experienced engineers,

so that they at least get a chance to know what they do not know.

LACK OF KNOWLEDGE OF MANY MATERIALS OF CONSTRUCTION

My students used to know about two materials of construction, which they used for

everything—carbon steel and stainless steel (they usually weren’t sure which grade).

There is rather more choice than that, as Table 10.1 shows.

LACK OF UTILITIES

Make sure all utilities are included at earliest stages, for example cooling water, nitro-

gen, and refrigeration as well as steam, process water, electricity, and compressed air.

If you are handling highly flammable materials, one way to make them safe is to

exclude oxygen from vessel headspaces with inert gas. Nitrogen is cheapest, though

sometimes more exotic gases are required. You need to make and/or store this on site.

LAYOUT

2D layoutBeginners to plant layout consistently fail to think in three dimensions—they lay pipe-

work and plant out on the floor in plain view in a way which renders it a dense series

of trip hazards, instead of fixing it to the walls or grouping in pipe racks and bridges

like real engineers.

Lack of room and equipment for commissioning and maintenanceThis is the layout version of steady state design myopia. Detailed consideration needs

to be given by the designer to how the plant will be accessed during commissioning

and maintenance activities. The safety implications of this make it a high priority.


Lack of control rooms and MCCsAs mentioned previously, new designers may be unaware that we need MCCs to control

the plant, and that we normally put these in a control room. The control room size

needs to take into account the direction from which the panel is accessed for mainte-

nance, the direction the cables come in from, and be big enough for safe access with the

MCC doors open. There will also normally be a table with a PC on for system control

and data acquisition (SCADA), room for filing cabinets for paperwork, etc.

PROCESS CONTROL

Lack of redundancy for key instruments and safety switchesBeginners tend to miss out key instruments entirely, and slightly more experienced

engineers can fail to allow for standby capacity for safety or process critical instrumen-

tation. Such standby provision needs to be balanced against the need for simplicity.

Lack of isolation for instrumentsInstruments need maintenance and replacement. Unless you only propose to do this

with the entire plant shut down and drained, isolation valves are recommended to

allow removal and replacement when the plant is running.

Measuring things because you can, rather than because you need toDon’t measure things you can’t control. It will only cost you money, and it might

upset you needlessly.

Alarm overloadConsider the number of alarms you are generating—don’t overload operators with more

alarms than they can take in. This will make the plant less, rather than more, safe.


P&ID notationWe mostly control plants with programmable logic controllers (PLCs) or distributed

control system (DCS) systems nowadays, so P&IDs should usually not show control

loops as if they were wall-mounted proportional, integral, differential (PID) control-

lers as shown in Figure 17.1:

FURTHER READINGSandler, H.J., Luckiewicz, E.T., 1987. Practical Process Engineering: A Working Approach to Plant

Design. McGraw-Hill, New York, NY.

PIC

PT

VFD

0–120PSI

Figure 17.1 P&ID notation.




CHAPTER 18

Design Optimization

INTRODUCTION

Premature optimization is the root of all evil.Donald Knuth

Contrary to popular academic opinion, availability of accurate information means

that design optimization is most usually and always best applied after a plant has been

built, commissioned, and operated for a while.

Design optimization tools such as modeling, simulation, and pinch technology are

therefore poorly suited to use during the plant design process, with a few

notable exceptions. Their use in academia is almost always misuse grounded in a lack

of understanding of the constraints of professional practice.


The main thing that those who apply academic process optimization techniques to

process plant design fail to understand is that the iterative nature of real design

processes means that there is already a complex design optimization process going on.

Professional engineers, however, understand that each stage of design has its own

natural resolution. There is no practical point in applying a technique with a

resolution finer than the model it is being applied to.

The second thing which the academic approach fails to address is that you cannot

meaningfully optimize a model which has not been verified by input of real-world

data.

Like microscopes, all design techniques have what we might call a limit of

resolution. Microscopic resolution allows us to distinguish accurately between two

lines. A microscope with insufficient resolution for the task to which we put it may

give the appearance of two lines where there is really only one, or one line where

there are actually two.

Similarly, a design technique with insufficient resolution may make two options

seem equal where one is actually better, equal options significantly different, or even

the better one worse. Misuse of process optimization tools for design is to me akin to

what is known in microscopy as “empty magnification,” where you make an image

look bigger, but it actually holds no additional information.


Process integrationWhat is known in academic circles as “process integration” is not design integration. It

probably isn’t even really process integration. The professional process designer’s “process

integration” balances a number of mutually dependent considerations. The design needs

to be safe, robust, and cost-effective, but safety and robustness do not come for free. A

balance has to be struck.

As piping and instrumentation diagram (P&ID), general arrangement (GA),

process flow diagram (PFD), process, and hydraulic calculations are developed, many

choices have to be made about the broad outlines of plant layout, degree of

redundancy of equipment, and basic approaches to safety.

Potential hazards have to be identified, quantified, eliminated, or controlled.

Materials and equipment have to be specified. While doing this, installation,

commissioning, maintenance, and nonsteady state operation of the plant have to be

considered. Past experience with other similar plants needs to be incorporated.

The process designer does not do this in a vacuum—they need to integrate the

requirements of and insights from other disciplines. Optimizing a few aspects of the

process, or even the whole process chemistry, is not optimizing the overall plant

design. It may actually be making it less optimal.

What is often meant by process integration in academia is use of a mathematical

analysis of a system using one of what is now a wide range of mathematical, graphical,

or computer-based tools, originally developed for beginners.

The problem these tools solve is one of handling a multiplicity of possible solutions. It

isn’t so much that there are an infinite number of possible solutions to the question, each

of which has a number of subtly different implications, as that there are a great number of

permutations to winnow for the best value of a single numerical selection criterion.

The tools can perform this winnowing process for us, but the fact that there is

essentially one right answer, and a computer can find it better than a person, tells us

that this isn’t really engineering, and the problem is essentially trivial.

These tools may have some limited use in the final stages of designs which use a

lot of energy, and have clear possibilities for substantial recovery of that energy. They

may also be of use in identifying possible improvements to existing processes.

Starting a design from heat integration of a process at steady state without consider-

ation of cost or other implications is trying to fit a job to a tool rather than the reverse.

Another buzzword in academia is “process intensification.” Professional engineers

make processes as intense as they practically can, but no more so, to paraphrase Einstein.


INDICATORS OF A NEED TO INTEGRATE DESIGN

Professional process plant designers always integrate their designs (it’s the most important

aspect of process design) though they are integrating different aspects than those using the

term in academic contexts. There are, however, certain contexts where they address the

same issues as the theorists, most notably where there are likely to be big cost

implications.

High utilities usage/wasteEngineers will always be concerned that their design might be less than optimal if

they see that a design has high operational costs due to high utilities usage.

I worked for some years for a UK government scheme called Envirowise in which

we visited process plants and factories to audit resource usage. What was clear to me

after doing a hundred or so of these visits is that there are a number of areas where

such wastage is commonplace. In fact Envirowise eventually gave us a table for the

best places to start looking, which I have reproduced later in this chapter.

The use of this table is far more economical in terms of designer resources than

carrying out a mathematical network analysis such as pinch. We have to conserve our

resources too.

High feedstock use/wasteFeedstock costs money, and the waste streams generated by wasted feedstock often

cost money to dispose of. This does not imply that the optimal feedstock conversion

rate is 100%. Each incremental increase in conversion usually costs more than the last

similarly sized increment.

Back when I worked for Envirowise we were encouraged to nudge people in a

direction toward zero waste, but there is almost always a point on that road beyond

which economic viability becomes questionable. It is, however, true to say that

uncritical acceptance of traditional levels of resource inefficiency is unlikely to yield

optimal design either.

A bit of analysis of resource usage is almost always informative and worthwhile for

operating companies. Designers should, however, already be taking these issues into

consideration using the standard combination of mass balance, appropriately accurate

costing and sensitivity analysis.

267Design Optimization

HOW TO INTEGRATE DESIGN

Since the mathematicians have invaded the theory of relativity, I do not understand it myselfanymore.

Einstein

Professional designers are employed to produce integrated designs, but the things

they balance are practical things like cost, safety, and robustness, process controlla-

bility and operability, and the conflicting demands of the various disciplines and

stakeholders involved.

They never optimize designs for a single variable or small number of variables, which

is why process plant design is more like playing chess than doing logic puzzles. In fact it is

far harder than chess, which is why people can still outdesign computers, but computers

are now the world’s best chess players (the fact that computers can do it better than people

shows that chess is in fact a very complex task, rather than a problem solving exercise).

There may be no right answer to a design exercise, but there are better and worse

answers, and better and worse players.

Those who think that process plant design is or will ever be a form of applied

mathematics simply do not understand the nature of design. They have simplified an

activity to a level where its point has been lost.

Intuitive methodMuch professional engineering knowledge is qualitative or semiquantitative.

Envirowise produced a number of graphs and tables to facilitate increased resource

efficiency of existing processes which designers can use to inform their design process,

which are reproduced in Tables 18.1�18.3 and Figures 18.1 and 18.2.

There are also useful tables on waste heat recovery in “Practical Process

Engineering” (see “Further Reading”).

These tables and charts are as useful to a beginning plant designer as an operating

company, as they show us where to look for improvement to our designs, and the

rough cost/benefit profile.


Table 18.1 Resource efficiency measures for process plantCost-effective water saving devices and practices for industrial sites: process planta

Item/Application

Description/Purpose Equipment/Technique Applicability Other benefits Otherconsiderations

Potentialcost

Potentialpayback

Liquid ring

vacuum

pumps

Reuse of sealing water

after treatment

Tanks/pumps/

separators/cooling

Widespread Energy savings

from cooling

seal water

Seal water:

temperature and

quality control

H M

Eliminate water use Mechanical vacuum

pumps

Widespread Liquid trap H M

Cooling

towers

Automatic

blowdown—

operation at

maximum

acceptable total

dissolved solids

(TDS) level

Conductivity-base

control

Widespread Reduced

chemical use

M S

Cooling load

reduction—minimize

evaporation and

blowdown

Widespread Reduced

chemical use

M M

Alternative cooling

processes to avoid

evaporation of water:

H M�L

(i) Air blast High cooled

water

temperature

(.40�C)

Monitoring

requirements

(ii) Heat exchangers Widespread Waste heat

could be

used

elsewhere

H M

Heat

exchangers

Water reuse through

closed loop system

Tanks/pumps/heating

source/cooling

source

Widespread Heat sink/

cooling

tower/water

quality

H M�L

Hydraulic

power

packs

Optimize water use

by varying water

flow depending on

oil temperature

Bulb-and-capillary

operate control

valves

Widespread Essential cooling

requirement

L�M S

Reuse after cooling—

through closed

loop system

Tanks/pumps/cooling

source

Large

installations

Cooling tower/

water quality

H L

Potential costs and paybacks are for guidance only. Actual costs and paybacks will vary due to project-specific details.aRisk assessment required; Potential cost: L5 low (minor alterations) (d0 to a few d100s); M5medium (a few d100s to a few d1000s); H5 high (extensive alterations or new plant required) (many d1000s)Potential payback: S5 short (a few months); M5medium (less than a year); L5 long (over a year).

Copyright material reproduced courtesy of WRAP/Envirowise.

Table 18.2 Resource efficiency measures for cleaning and washdownCost-effective water saving devices and practices for industrial sites: cleaning and washdowna

Item/Application

Description/Purpose

Equipment/Technique

Applicability Other benefits Otherconsiderations

Potentialcost

Pressure

control/flow

restriction

Reducing

instantaneous

flow at point of

use

Valves, orifices,

pressure-reducing

valves

Variable or

intermittent

supply, pressure

or demand

L S

Countercurrent

rinsing

Reuse of rinse

water

Tanks Multistage unit

processes

Water quality

requirements

M L

Spray/jets Appropriate

application of

water

Nozzles Widespread Improved

cleaning

L�M S�M

Spray nozzles Widespread Improved

cleaning

Spray mist drift L�M S�M

High pressure spray

packages

Washing processes Improved

cleaning

Power

consumption

M�H S�M

Automatic

supply

shutoff

Use of water only

when needed

Solenoid valves in

pipelines

Small bore pipes Essential water

requirement

L�M M

Actuated valves in

pipelines

Large bore pipes Essential water

requirement

M M

Jets/ spray guns on

hoses

Widespread More

efficient

application

Theft of spray guns L S

Reuse of wash

water

Reuse of wash

water in other

areas

Tanks/pumps Widespread Cross-

contamination/

water quality

control

M S�M

Scrapers/

squeegees/

brushes

Sweeping up of

slurries

Dry cleaning

methods

Large areas Possible reuse

of

materials

Dry collection

systems

L S

Cleaning-in-

place (CIP)

technology

Countercurrent

reuse of rinse

water with

multiple reuse of

chemical

cleaners

Proprietary plant Processes with

frequent cleaning

Hygienic

plant/

minimal

downtime

for

cleaning

Water quality

requirements

H S�M

(Continued)Cost-effective water saving devices and practices for industrial sites: cleaning and washdowna

Item/Application

Description/Purpose

Equipment/Technique

Applicability Other benefits Otherconsiderations

Potentialcost

Recycle after

treatment

Treatment of

wastewater to an

acceptable

standard for

reuse

Filtration/

sedimentation

Coarse solids

removal/phase

separation

Waste disposal and

water quality

M M�L

Centrifugation/

flotation

High quality solids

removal/phase

separation

Waste disposal and

water quality

H M�L

Biological

treatment

Removal of

dissolved

biodegradable

solids

Waste disposal and

water quality

H M�L

Ion exchange Removal of

dissolved

contaminants

Waste disposal and

water quality

H M�L

Distillation/

stripping

Solvent recovery By-product Waste disposal and

water quality

H M�L

Absorption/

adsorption

High quality

treatment, solvent

recovery, removal

of toxic

substances, color,

etc.

Disposal of spent

absorbent

H M�L

Potential costs and paybacks are for guidance only. Actual costs and paybacks will vary due to project-specific details.aRisk assessment required; Potential cost: L5 low (minor alterations) (d0 to a few d100s); M5medium (a few d100s to a few d1000s); H5 high (extensive alterations or new plant required)(many d1000s); Potential payback: S5 short (a few months); M5medium (less than a year); L5 long (over a year).


Table 18.3 Typical water savingsWater saving initiative per project Typical reduction per site

Commercial applications

Toilets, men’s toilets, showers, and taps 40% (combined)

Industrial applications

Closed loop recycle 90%

Closed loop recycle with treatment 60%

Automatic shutoff 15%

Countercurrent rinsing 40%

Spray/jet upgrades 20%

Reuse of wash water 50%

Scrapers 30%

Cleaning-in-place (CIP) 60%

Pressure reduction See Fig 18.1

Cooling tower heat load reduction See Fig 18.2


6

5

4

3

2

1

00 2 4 6 8 10

Percentage reduction in water distribution pressure

Red

uctio

n in

wat

er u

se a

tje

ts, n

ozzl

es, a

nd o

rific

es (

%)

Figure 18.1 Effect of pressure reduction on water use at jets, nozzles, and orifices. Copyright imagereproduced courtesy of WRAP/Envirowise.

Percentage reduction in heat load on cooling tower

00

5

10

15

20

5 10 15 20

Red

uctio

n in

mak

e-up

wat

er r

equi

rem

ent (

%)

Figure 18.2 Effect of heat load reduction on make-up water requirement for a cooling tower.Copyright image reproduced courtesy of WRAP/Envirowise.

Formal methodsPinch analysisOriginally intended for optimizing heat recovery, pinch analysis has also been applied

to analysis of mass flows, including water flows. It is neither novel, nor much to do

with professional design, but academics love to apply it to design. I learned it in uni-

versity, and not only did I never use it, but I never once heard it mentioned until I

entered academia again 20 years later.

An outline of the most commonly used method for the production of water purity

profiles with a fixed flowrate for a single contaminant is as follows:

1. Draw a graph of flow rate versus concentration for all sources and sinks of water

on a plant, where x-axis is flow, ascending from zero, and y-axis is concentration

of contaminant, descending from zero.

2. Start on the left plotting flow rate/concentration pairs for potential sources of

water, in increasing order of purity (dirtiest on the left).

3. Join the points to form a stepped “curve.”

4. Next plot flow rate/concentration pairs for potential sinks and join the points to

form a second “curve.”

5. Move the sinks curve to the left until the curves just touch.

6. Where the two curves touch is the pinch point.

7. Where these two curves overlap represents the scope for water reuse.

8. Area to the left of any overlap represents wastewater generation.

9. Area to the right of any overlap represents clean water use.

Next, we can use sensitivity plots of potential cost saving versus concentration

change for multiple contaminants, to identify areas where variation in maximum

allowable inlet and outlet concentrations would yield the greatest savings.

We can consider four possible levels of investigation of the water reuse possibilities

while carrying out our pinch analysis.

The cheapest and quickest analysis assumes that all sinks are presently at their

maximum allowable inlet concentrations. This level of analysis will identify some

cheap modifications which will yield small benefits.

Next we might consider the possibility of increasing the maximum allowable inlet con-

centrations in those areas where our sensitivity analysis has indicated that large savings might

be available. There are technical limits on how far these concentrations can be increased

without causing corrosion or other problems, which should be established and considered.

Greater savings are likely to be obtained by this more in-depth analysis than for the simpler

one above, and the costs of identified modifications are likely to be quite low.

A more rigorous analysis still considers the possibility of reuse after regeneration

by water treatment technologies of a number of key streams. This can involve

significant expenditure on water treatment plant.


Similarly, we might consider distributed effluent treatment techniques—rather

than mixing all effluents together prior to treatment, we can consider treating or

partially treating wastewater streams individually. It is claimed that this technique can

offer improved contaminant removal efficiency at reduced cost.

Note that we need meaningful data on water quality and quantity produced and

used to even start this process. People operating a real plant can obtain such data, but

it is not available to plant designers. The same is true of all other pinch analysis tech-

niques, which is the first reason why real process plant design engineers don’t use

it—they can’t.

WHEN AND HOW NOT TO INTEGRATE DESIGN

Pinch analysis has no part in professional design exercises, and its inclusion in the design

process would not merely be an extraordinary waste of time. Its use necessarily leads to

suboptimal design. Amusing as it might be as an academic exercise to play with novel

but unrealistic approaches, the downside of pinch analysis being the starting point for

plant design is so obvious to professional engineers than no one ever does it.

WHERE’S THE HARM? THE DOWNSIDE OF ACADEMIC“PROCESS INTEGRATION”

Capital cost of “integrated” plantsWhen applied to energy use, the marginal cost/benefit ratio of each additional heat

exchanger is not really considered by many of the techniques used by pinch analysis

enthusiasts. This means that they are likely to be recovering heat at a greater cost than

it can be purchased for. Real engineers don’t do that.

It can be seen from the Envirowise table that the cost of additional heat exchangers

is rated as “high,” but that the potential for payback is only medium�high. Every

additional increase in energy recovery needs to make financial sense. These very sig-

nificant costs are not taken into consideration by process integration techniques as

applied to design.

Commissioning “integrated” plantsThe integration of heat exchanger networks means that the plant produced would be

normally operated in a highly interdependent manner, and (theoretically at least), with

reduced energy inputs.

The process commissioning engineer will have to get such a plant from an initial

condition, in which even single systems are not locally balanced with respect to their

flows and control loops, to one in which there are extensive cross system mass and


energy balances. This is going to take additional time and other resources in the most

resource-pressured part of the job.

If the designer has not included heating and cooling services capable of providing

the full heat loads, ignoring integration, the commissioning engineer will need to

make provision for them.

Small incremental savings in energy recovery may easily save less over the plant

lifetime than the extra commissioning time and resources cost. These very significant

costs are not taken into consideration by process integration techniques.

Maintaining “integrated” plantsMaintenance of integrated plants will carry the same problems as commissioning, and

all the additional equipment will have its own additional maintenance requirements.

These very significant costs are not taken into consideration by process integration

techniques.

FURTHER READINGAnon, 2005. GG523 Cost-effective Water Saving Devices and Practices—For Industrial Sites.

Envirowise.Sandler, H.J, Luckiewicz, E.T., 1987. Practical Process Engineering: A Working Approach to Plant

Design. McGraw-Hill, New York, NY.




CHAPTER 19

Developing Your Own Design Style

INTRODUCTION

There is a great deal more to engineering than the stuff they teach you in university,

but I’m not talking about the ethics and embedded humanities modules which some-

times get shoehorned into curricula. There are even a few useful books on the subject.

THE ART OF ENGINEERING

Engineering design is not applied science. It is an art, learned and refined through prac-

tice. Engineers use all that they are in the practice of their profession. Intelligence and

knowledge which are neither scientific nor mathematical are of crucial importance.

When I teach design to mature postgraduate students, it can seem as if they are more

creative than the undergraduates, as they come up with a lot more ideas than the students

with no industrial experience (but higher entry qualifications). I do not think that this is

pure creativity. I think it is that they have more life experience to be creative with.

Judgment, intuition, and the knowledge and experience which teach us what

doesn’t work and enables us to reason by analogy all take time to develop.

Back when I was learning to teach I wrote a blog reflecting on my experiences,

from which an excerpt follows:

I went to see a client today. He had a problem, and had changed five things which mighthave caused it, as well as several others which might not (though he didn’t understand that).I knew which two were the causes in ten minutes.

I think to myself: 1. Engineering is easy (for engineers). 2. How did I learn how to do that?How can I teach others to do it? I could try this example as a case study, and see how hard itis to people earlier in their training. It seems to me at present that more so than amassingfactual knowledge, it’s to do with acquiring the engineer’s perspective. Whilst it may have itslimitations, on its home turf, it can cut through confusion, obfuscation, and misunderstandingin a flash.

There is, however, no substitute for having a firm grasp of practical math and physical sci-ence. Theory underpins practice, and is available for verification of intuitive understandings. Anexperienced professional is not necessarily doing math and science in their heads when trou-bleshooting a problem. It is more like pattern-matching “Oh, yes, this reminds me of that timewhen. . .” and not necessarily even the words, just seeing into the problem, pruning the tree ofpossibilities. This involves people, and discourse (though engineers do not call it that).

I spent far more time talking to the maintenance technician yesterday than I did lookingat the machinery. In talking to him, I have to get him to talk freely, so that he tells me whathe thinks has been happening. I have to assume that he will see what he expects to see, and


make sure I trust nothing of what he says which I have not verified personally. I look for areaswhere what he says is self-contradictory, and explore those areas with him in a way whichdoes not make him feel I am trying to trap him into admitting he screwed up, or rubbish hispet theory.

I am, however, quite ruthless in making sure that I get to the bottom of what is happen-ing to my own satisfaction. I’m going to find out what is wrong, and I’m going to fix it. Howmuch I tell his boss is, however, negotiable. He knows how I work, since we have been inter-acting for a year or so, and we conduct an unspoken negotiation between us.

I am commercially interested in extending and upgrading the plant, but am constrainedby professionalism not to milk the client. He is paid to maintain the plant, but would like it tobe as automated and reliable as possible to make his job as easy as possible. He is, however,also paid to minimize costs, consistent with meeting the required effluent quality.

Between us we come up with a plan which makes us both look good, him cost consciousto his boss, and the client actively addressing the effluent failures from the point of view ofthe authorities. It also has a bit of what both he and I want, which is to pay me to make theplant better from the point of view of the maintenance staff as well as the other parties.

I’ll also put some nice things in my report to his boss about the build quality of the modi-fications he has made and underplay their contribution to the problems. He in return will notmistreat the plant in between visits and blame it on my design. All of this is unspoken, but Iknow it is going on, and I think he does too.

Away from science and engineering, they might think chemical engineering is all aboutnumbers and chemicals, but it seems likely to me that professional practice has less of thisthan academia. Working with other people’s fears and desires, their wishful thinking and self-deception, strengths and shortcomings (as well as our own) is a crucial part of the job-butthat doesn’t mean a psychologist could do it.

THE PHILOSOPHY OF ENGINEERING

Ultimately the philosophy of engineering is as useful to engineers as philosophy of sci-

ence is to scientists. “Philosophy of science is about as useful to scientists as ornithology is to

birds” as Richard Feynemann is supposed to have said.

You might, however, have noticed that I have read quite a few books on the phi-

losophy of engineering for someone who dismisses philosophy so readily, but I have

only recommended books on the philosophy of engineering written by engineers.

When I came to have to teach others what I had learned, but no one had taught me,

I needed to figure out what I knew and how I knew it to be true.

It seemed before I read these books that figuring out what I knew about engineer-

ing and how I knew it was the subject of philosophy of engineering. However, it

turned out that the subject of philosophy of engineering was philosophy, not engi-

neering. I did find out one useful thing though, which was that much of what I knew

was far from scientific.

Until this point I still thought that engineering was applied science. Anyone that

has actually practiced the profession and had time to reflect upon it will see that it is


far more art than science (though those who have never practiced, and those who

have never reflected upon their practice might disagree).

So philosophy of engineering might be useful after all, as a way to get nonpracti-

tioners to understand in an abstract intellectual way that engineering’s foundation is

not science and math, but praxis, the process of design.

This intellectual knowledge would not, however, be the fruit of praxis itself, and

an academic informed by philosophy as to the nature of engineering would be a phi-

losopher who understood what engineering was rather than an engineer. They might

try to remember that if they write books on engineering.

THE LITERATURE OF ENGINEERING

I read a lot of books on process design in preparation for writing this book, and most

of them were worthless. Books on design by people who had never designed anything

(whether they are philosophers or any other kind of academic) are as accurate and

informative as a braille list of rainbow types written by a person who had only ever

heard them described.

I did not read those books to learn what process design is. The approach given in this

book is not derived from the books I recommend, I have merely recommended those

books which seemed to be informed by an understanding of what design is about.

Of far greater use to engineers than philosophizing are readable books which

move our understanding of the nature of engineering forward. We can actually learn

what doesn’t work and why it doesn’t more efficiently from books than from practice.

I would recommend, at a minimum, reading:

Trevor Kletz, An Engineers View of Human Error (or anything else by him).

Henry Petroski, To Engineer is Human (or almost anything else by him, though later books arenot so good).Harvey Dearden, Professional Engineering Practice: Reflections on the Role of the ProfessionalEngineer.

If you are in a situation later in your career where you need to understand what

you know without getting lost in abstraction, I would recommend:

Walter G. Vincenti, What Engineers Know and How They Know It.

Eugene S. Ferguson, Engineering and the Mind’s Eye.Donella H. Meadows, Thinking in Systems.Billy Vaughn Koen, Discussion of the Method.

THE PRACTICE OF ENGINEERING

Accept no substitute, for there is none. Psychologists disagree with each other about

the relationship between practice and mastery, but only because many of them aren’t

279Developing Your Own Design Style

really scientists. They think it means that practice may be irrelevant to mastery if

some never achieve mastery despite great amounts of practice, and some achieve mas-

tery with very little (reported) practice. People lie, and anyone who teaches anything

can see that some people have little aptitude for their subject.

Whatever your starting point, there is no substitute for persistence, as Calvin

Coolidge said:

Nothing in the world can take the place of persistence. Talent will not; nothing is more com-mon than unsuccessful men with talent. Genius will not; unrewarded genius is almost a prov-erb. Education will not; the world is full of educated derelicts. Persistence and determinationalone are omnipotent. The slogan Press On! has solved and always will solve the problems ofthe human race.

Some will find during practice that they are practicing the wrong thing, and

change course, and there will always be an advantage to those with talent, but those

who persevere will become Engineers.

Like Casey Ryback, I also cook, and I find practicing engineering is far more like

cooking than it is like doing exam questions. If your educational assessment only con-

sisted of completing exam questions you might find that what you are good at is

exams rather than engineering.

PERSONAL SOTA

Koen calls the set of heuristics used by an individual their sota (for state of the art),

and uses quasi-mathematical notation to show it as being that of an individual at a

given time thus: sota |sean moran; 2014 would be mine as of now. This may be a little

gimmicky, but it does allow him to illustrate the relationships between individual sotas

and best practice as the intersection on a Venn diagram of all sotas. So this book is a

subset of sota |sean moran; 2014, which I have gone to some effort to ensure corresponds

with current best practice, sota |Eng; 2014.

My sota depends upon my background, as does yours. Engineers are not

generic—there are reasons why I am the kind of engineer I am (feel free to skip this

bit if you think it self-indulgent).

All of my siblings are engineers. My father was not a professional engineer, but he

was a pipefitter and mechanic who worked in engineering, specifically the operation

and maintenance of power stations.

I have been hearing the stories engineers tell of what works and what does not

along with explanations of the bits of kit being discussed all of my life. I also heard

the stories maintenance staff tell about the folly and arrogance of green professional

engineers who think book-learning alone is superior to experience and “technician

level knowledge.”


We didn’t have a lot of money when I was a kid, and I had to learn how to mend

my own stuff, as well as help my dad mend the car and do on. I also used to take apart

any bits of machinery I found to see how they worked, and when I wanted a bike, I

built more than one from discarded bits. I also built myself various types of compu-

ters. So I went into education already knowing quite a lot of useful things about the

nature and culture of engineering, and with hundreds of hours of fiddling with engi-

neered products.

A very un-PC technician got us all together when we covered process technology

in my first degree, and said to us all “those of you who like fixing your own cars and

bikes are going to do well in this course. Those of you (and I’m talking about the girls

and Asians here) who don’t aren’t going to like it at all”. He was wrong about women

and people of Asian descent, but he was right about the link between a history of tin-

kering and a feel for engineering.

I am, by original education, an applied biologist. Biology was more about classifi-

cation than deep understanding back when I started studying it. Molecular biology

has elucidated the mechanisms behind much that was obscure back in the 1970s, but

despite progress in systems biology, multicellular life is still very far from complete

explanation. The subject is still mostly about the readily observed but ill-explained

emergent properties of irreducibly complex systems.

The most important math in biology is still statistics, and often the nonparametric

statistics of populations which do not meet the assumptions underlying the more

commonly used kinds of statistics. Biologists understand that statistics is itself a set of

heuristics, true only if their underlying assumptions are true. The most commonly

used kind of statistics are not very robust, but are frequently used where their underly-

ing assumptions are not true, rendering them at best meaningless. There is no such

thing as 2.4 children.

Biology is an essentially qualitative field of study, whose objects are complex bio-

chemical and physical processes controlled by homoeostasis, in which variables are

regulated so that internal conditions remain stable and relatively constant. Despite this

vagueness, we have for a very long time been able to make biology do things that we

want it to.

So for me, a process plant is like a biologist’s plant. I don’t worry about whether I

understand every aspect of the reductionist science of its subcomponents, or even

whether it is possible to do so. They are unimportant to the job of engineering a plant

which will give me the yield I want of product in a safe, reliable, and cost-effective

way.

Then there is the element of chance and the opportunities available to us—I origi-

nally trained as a biochemical engineer, and ended up in water treatment plant design

due to unexpected political resistance to biotechnology, and the cyclical nature of the

water industry. My process plant design experience was mostly with contracting

281Developing Your Own Design Style

companies, and my job title was proposals engineer, rather than process engineer. I

had to design whole plants, coordinate all disciplines, and care a great deal about cost,

risk, footprint, and so on. I think this is why this book has a different scope from

other professionals whose job title was or is process engineer.

Such books tend to be limited in scope to very detailed design of unit operations,

because that is what process engineers are often asked to do. Engineers of other disci-

plines (notably mechanical) coordinate the design effort, and process engineers are just

used to carry out the chemically bits the mechanical engineers cannot do. This is my

working explanation for why process plant design books by process engineers are nar-

row, deep, and limited in scope, and the best book I could find on the subject was

written by mechanical engineers.

Then there is the question of attitude—I started out overconfident, nearly won a

job which I would have regretted winning, and became more cautious. I have seen

overcautious engineers tread a reverse path. Experienced professional engineers still

differ from each other in their risk aversion, but both overconfidence and its opposite

are corrected by experience in those who stay in engineering. Other personality traits

also often tend toward the middle way as a result of practice.

Your background and experience will be different, but practice will shape you into

a particular kind of engineer. You will have a personal state of the art. You will find

yourself seeing all kinds of problems away from professional life as soluble with the

tools of engineering. You will come to understand that in life, as in engineering “all

is heuristic,” as Billy Vaughn Koen says. Until then you’ll have to take my word for it.

FURTHER READINGKoen, B.V., 2003. Discussion of the Method. Oxford University Press, New York, NY.



APPENDIX 1

Integrated Design Example

I often use a particular Siemens ultrasonic level instrument to measure tank fluid

levels. The instrument can display just the distance from ultrasonic transducer to liquid

surface, or it can be programmed to display fluid level or even fluid volume (even

with quite complex tank shapes).

It can output fluid level or volume as a 4�20 mA signal. It has five volt-free con-

tacts which I can program to become active or inactive at various tank levels. It can

have two transducers and be programmed to make decisions about which transducer’s

signal is the more reliable. It has all kinds of sophisticated signal processing onboard,

and can be programmed as to which state to fail to if the signal from transducers is

lost. It can produce alarm signals against many of these functions.

So if I want to control a set of duty/standby pumps in such a way as to maintain a

constant tank level, and to avoid dry running, I have choices. The way I tended to

use this instrument, back when it was less sophisticated, was to use it as a field-

mounted standalone on/off level indicator controller. I used the volt-free contacts to

start and stop pumps in succession, with the lowest set point providing dry running

protection. The pumps were started under control of these contacts by online or star

delta starters, and were either on or off.

I tend nowadays to use the instrument in one of two ways:

1. As a more sophisticated standalone modulating level indicator controller. I can

wire the 4�20 mA output signal directly into a pump’s inverter starter. Even

motor starters have built-in computers nowadays, and an inverter drive can use the

4�20 mA signal to achieve all the same control actions as the last approach with

no programmable logic controller (PLC) involvement.

2. As a dumb level indicator transmitter, with the 4�20 mA signal going out to the

PLC, which then controls a number of pumps using variable speed drives (inver-

ters) to go faster or slower in order to maintain a fairly constant tank level. I will

probably under this scenario use one of the volt-free contacts as my hard-wired

(direct into the pump starter) dry-running protection.

There are many other ways to use the instrument, but to take the three I have

covered, the first and second have no PLC involvement, so they may save money on

PLC costs, but with the drawback that they are less flexible as they cannot be so

readily controlled via the PLC or system control and data acquisition (SCADA)

system.

283

There is only one 4�20 mA output on the instrument, so if we wire it into the

inverter, we cannot straightforwardly supply it to the PLC, so we cannot remotely

monitor tank level, or intervene to alter action levels, and so on.

(A C1 I specialist comments: “4�20 mA can be routed through more than one

device as current loop; better practice is to use resistor to convert to voltage and paral-

lel connect voltage signal into as many devices as you like. Inverters might have out-

put available that could be configured as repeat. There are repeater isolator devices

available.”)

As I said, “cannot be so readily controlled.”

So in this very common design scenario, which features at least once on virtually

every plant I have designed, I tend to choose between these three options which I

have used dozens of times before unless I can see a strong reason not to.

There is nothing wrong with starting the detailed design of process control systems

by using standard control loops, strategies, and instruments, as it is close to how expe-

rienced professionals design. We have well-tried strategies in our heads, in much the

same way as chess players do. Any inappropriate elements or unforeseen interactions

between stock approaches should be picked up in design reviews and Hazard and

Operability (HAZOP) studies.

At the design for construction stage, there are many interactions between the ele-

ments of design which experienced engineers will have anticipated at earlier design

stages. In the example given, we are considering pumps and electromechanical com-

ponents in addition to the software, controllers, and instruments which commonly fall

under the heading of process control. We are informed more by broad experience

than by a narrow but mathematically rigorous analysis of a few parameters.

INTEGRATED PROCESS CONTROL AND DESIGN EXAMPLE

To illustrate how we balance qualitative considerations, I will take an example from

the water industry which I am very familiar with.

Consider the case of pretreatment of filter feed water. We need to add a coagulant

to allow for the removal of colloidal matter, measure pH (and temperature to adjust

for its effect on pH), and adjust pH to the minimum solubility of the product of our

coagulant addition.

In theory this is very simple, but consider the following issues affecting process

design/control:

Conceptual design issues• The coagulant dose can be fixed, dosed proportional to flow, or dosed propor-

tional to flow and color.

284 Appendix 1: Integrated Design Example

• Many coagulants are strongly acidic, so the coagulant dose will affect acid/alkali

dose.

• We can choose lime or caustic for alkali addition. As lime has a greater buffering

capacity, has an appreciable reaction time, and needs to be kept stirred if it is to be

reasonably homogeneous, caustic is usually more controllable, but less forgiving of

long loop times.

• We usually use hydrochloric or sulfuric acid for our acid addition. Adding chloride

to the system may require a higher material specification.

• We usually need to control pH to within 0.1 pH units to give sufficient control

over coagulant product solubility.

• We can dose chemicals using a number of pump types, or by gravity via a control valve.

• We need to have a short, sharp mix for both dosed coagulant and any required pH

correction acid/alkali.

• We need to have a longer lower intensity mix for growth of the floc particles

which the filter will remove.

• We can use static or dynamic mixing for either of these duties.

• Static mixers may not achieve the specified degree of mixing in the mixer body—

the measurement point might need to be several pipe diameters downstream.

• Static mixers for chemical addition have a hydraulic residence time (HRT) mea-

sured in seconds. Dynamic mixers can have a HRTof several minutes.

• Both static and dynamic mixers for flocculation have a HRTof several minutes.

• There is no field-mounted instrument which reliably measures efficiency of floc-

culation, though we can measure it indirectly by measuring the turbidity which

escapes the filter downstream, several minutes later.

Layout/piping issues• We may need to have a certain length of straight pipe after the static mixer before

the downstream pH sensor to ensure accuracy.

• We need to make sure the flow meters will run full, which may require their

installation at a low point (avoiding dead legs), to ensure accuracy.

• The pH probe needs to be regularly removed and replaced for calibration, so it

needs to be installed in such a way as to make this easy for operators.

• If we are going to use a gravity chemical addition system, we have to have a cer-

tain height available to us between storage tank and dose point.

• The output from the flocculator should be subjected to minimum shear prior to

filtration to avoid breaking flocs.

• If we are going to use open-topped dynamic mixers, we will break head, so we

need to arrange in the layout for them to flow to each other and then on to the

filters by gravity, or (less favored) we need to add intermediate pumping stages.

285Appendix 1: Integrated Design Example

Dosing issues• Positive displacement pumps give a pulsating flow.

• Static mixers only mix radially.

• As a consequence of the last two issues, pulsation damping is strongly recom-

mended if piston diaphragm (PD) pumps dose into a static mixer.

• Saturated sodium hydroxide solution freezes at 10�C.• Precise control of chemical flow via a control valve on a line from a header tank is

very hard to achieve, so we need to measure dosed flow if we are dosing via a con-

trol valve.

• In very dirty applications it might be difficult to keep the pH probes clean.

• Many pH probes now come with onboard temperature correction.

• Cheap and reliable field-mounted dedicated pH controllers are readily available.

• PD pumps are available which allow control of motor speed by one input

4�20 mA signal and control of stroke length via another.

Price issues• Static mixers are far cheaper to buy and run than dynamic mixers.

• PD pumps are far more expensive to buy and run than solenoid pumps.

• Field-mounted pH controllers are a cheaper way to control pH than PLC control.

• Electromagnetic flow meters are more expensive to buy than turbine type, but are

cheaper to run.

Safety issues• Header tanks full of acids and alkalis at height carry safety concerns.

• The pressurized ring mains at height used to fill these header tanks carry safety

concerns.

• We need to be sure that our main process pipework will not fill up with acid or

alkali when the main plant shuts down, and that even if this did happen, this

would not result in loss of containment.

• We need to be sure that positive displacement pumps are not throttled on suction

or delivery sides, and that if making this impossible is unavoidable, over pressuriza-

tion of delivery pipework does not lead to loss of containment.

• We need to be sure that even if our efforts to prevent loss of containment of acid/

alkali fail, we have secondary containment in place.

• We need to be sure that if loss of containment occurs, and operators are contami-

nated, they can wash off the chemical ASAP.

• We need to be sure not to mix acid and alkali with each other.


Robustness issues• If the operation of our plant depends upon effective coagulation, we will need

standby units for all coagulant and acid/alkali dosing systems.

• PD pumps have a much better turn down, and are far more controllable than sole-

noid pumps.

• Peristaltic pumps are better than piston pumps for lime dosing, but their hoses

present maintenance problems.

• Chemical addition is usually done via flexible lines. These lines have a limited life in ser-

vice, and consideration needs to be given to their repeated replacement in operation.

• Systems based on dynamic mixing have far longer response times due to their

much higher HRT.

• Electromagnetic flow meters are far more reliable and less prone to blockage than

turbine type.

• Electromagnetic flowmeters are far more precise than turbine type.

• Flocculation can to some extent be optimized by measuring zeta potential, and

particle size analyzers to measure filtration efficiency. However, in my opinion the

kit to do this is, presently, more suited to lab than field (others disagree).

• We need to ensure dry running protection for any dosing pumps.

• We need to be able to tell if any powered mixers are operational.

• We need to know if any control valves are actually in the position we have

requested.

• We need to know that the main process stream is flowing in order to prevent dos-

ing into an empty line.

• Some flowmeters have empty pipe detection built in.

Integrated solutionSo I would personally use a PD pump, with a 4�20 mA (or sometimes pulsed) signal

from a field-mounted pH controller controlling stroke, and a 4�20 mA signal from an

electromagnetic flowmeter in the main flow (with empty pipe detection) controlling

speed for acid/alkali addition.

I would usually use caustic and sulfuric acid at around 40% w/w solutions for pH

correction, though I sometimes use lower concentrations for operator safety reasons

and to make freezing in the absence of tank heaters less likely.

The pump will deliver against a loading valve, and in between the loading valve and

the pump there will be a suitably specified (especially in materials and capacity) pulsa-

tion damper vessel. Between the pump discharge and the first valve, there will be a

return line to the feed tank protected by a pressure relief valve (PRV). This arrangement

shall be so arranged that if the PRV lifts, the return flow will be visible to operators.

287Appendix 1: Integrated Design Example

The pH signal in to the controller comes from a probe, mounted five pipe dia-

meters of unobstructed pipe downstream of a static mixer, specified to give CoV

(Co-efficient of Variance)5 0.05 (or less technically a 95% degree of mixing) at that

measurement point across the operating flow range.

The static mixer has separate connections for coagulant, acid, and alkali. The che-

micals are dosed through injection lances (incorporating spring-loaded nonreturn

valves) into the centerline of the pipe. Hoses should be contained in trace-heated and

lagged pipes which provide a duct to pull though a replacement hose, as well as

secondary containment.

Coagulant dose is via a PD pump (or a solenoid pump if the plant is small and

price is crucial), controlled by a 4�20 mA signal direct from an electromagnetic

flowmeter.

Chemicals will be stored in quantities suitable for at least a month, with external

connections suited to suppliers working in that area. In the United Kingdom we

might need a tank heater, and will probably need trace heating and lagging of NaOH

systems at a minimum. Ideally all lines which might freeze under all reasonably fore-

seeable conditions will be trace heated and lagged. (I have had clients talk me out of

this requirement in the past, and they have had cause in time to regret the

penny-pinching.)

The dosed main process flow will pass from a static mixer to a static flocculator,

selected for robustness and simplicity. Line sizes and fittings from flocculator to filter

shall be selected to minimize headloss and therefore shear.

(I have been told by a correspondent that digital dosing pumps with integrated

speed/stroke control have replaced simple PD pumps—it only takes a few years for

the state of the art to move on).

So Luyben’s idea of incorporating controllability into design is sound, but there is

a lot more to controllability than math or generic theory. The insights of plant opera-

tors and commissioning engineers and their knowledge of the detailed characteristics

of kit need to be incorporated into the design.

Some might think that some of the things in the description I give above are not

control issues, but they encapsulate choices as to whether to solve a design problem

with soft or hard control. For example, if I do not ensure that my pipes will not freeze

through physical arrangement, soft controls will be needed.


APPENDIX 2

Upset Conditions Table

This table, and the associated text, was first published in “Process Plant Design and

Operation,” Doug Scott & Frank Crawley, pp. 94�105, Copyright Elsevier 1992.

SPECIFIC UPSET CONDITIONS

In the pages that follow, upset conditions are identified by the guidewords used in

hazard and operability studies—this is where upsets are most likely to be identified

prior to plant operation. The upset condition can then be analyzed under the four

phases of the project: conceptual (Con); detailed design (Det); startup (St), and opera-

tion (Op). For ease of presentation and analysis, the conditions are analyzed in block

format with possible solutions and individual reference numbers. The time when the

upset condition is most likely to be detected is indicated by an “X” in the project

phase. In this manner, the upset condition can be read vertically in the matrix and the

project timing can be read horizontally.

It should be noted that the proposed solution may not always be applicable to a

specific problem, but they all have been used at one time or another. Any tabulation

of this type can only give guidance. It can never hope to be comprehensive. In some

instances conflicting requirements may be identified and engineering judgment will

be necessary to ensure the most appropriate solution is chosen.

Table A2.1 Specific upset conditionsPressure

No Condition Solution Most likely stage

Con Det St Op

P1 More pressure Devise a process which operates at or near

atmospheric pressure and does not

utilize volatile fluids or vigorous

reactions.

X

P2 More pressure Specify the design pressure of equipment

such that it cannot be overpressured by

any condition other than fire.

X X

P3 More pressure Consider the potential for metal fatigue

following pressure cycling.

X X

(Continued)

289

Table A2.1 (Continued)Pressure


Con Det St Op

P4 More pressure Take due account of elevation changes

and design for the sum of both

hydraulic head and vapor pressure

(see L1).

X X X

P5 More pressure Install a high integrity protective system

to shut the process down before the

overpressure condition is encountered.

X X

P6 More pressure Install a full rated safety relief system and

analyses the means by which the fluids

may be dispersed in they are toxic or

flammable.

X

P7 More pressure Steam trace or, purge relief valve nozzles

to prevent the deposition of foulants.

X X X

P8 More pressure Specify the failure action of control

systems so as to minimize the effect of

failure (see P16).

X X X

P9 More pressure Initiate control procedures such that flow

limiting devices such as orifice plates

and control valves can only be changed

after a safety study has been carried out

(see F4 and OP2).

X X

P10 More pressure Carry out routine proof tests of relief

valves and test facilities for protective

systems.

X X X

P11 More pressure Install duplicate relief valves and test

facilities for protective systems.

X X

P12 More pressure Rod through vents to ensure that lines are

clear of debris. Check flame arrestors

are clear and not choked with debris

(see F8, F9 and P21).

X X X

P13 More pressure Install purge points on total condensers to

allow the removal of inert gases like air

and nitrogen.

X X X

P14 No pressure Choose a process which will not reach a

hazardous condition if pressure falls or

is lost; this may apply to oxidation

processes where oxygen and

hydrocarbons could enter the

flammable regime should the reaction

stop.

X

(Continued)

290 Appendix 2: Upset Conditions Table

Table A2.1 (Continued)Pressure


Con Det St Op

P15 No pressure Specify the metallurgy such that the metal

will not enter a brittle regime when

depressured. This might apply to

cryogenics and refrigerants (see T13).

X

P16 No pressure Specify failure action of control systems so

as to minimize the effect of the failure.

This may be contrary to the needs of

more pressure (see P8).

X X X

P17 Less pressure Consider the effects of leaks in vacuum

condensers. This is a variant of no

pressure.

X X X

P18 Reverse

pressure

Design equipment for full pressure and

vacuum, including liquid head.

X

P19 Reverse

pressure

Specify pressures within the process such

that leakage across heat exchangers

produces a safe condition, e.g. steam

leaks into hydrocarbons and not

hydrocarbon leaks into steam.

X X

P20 Reverse

pressure

Install vacuum protection where

appropriate e.g. on fixed roof storage

tanks.

X

P21 Reverse

pressure

Rod out vents to prove they are clear.

Check flame arrestors for debris (see

P12, F8 and F9).

X X X

P22 Reverse

pressure

Check vacuum relief valves for operation. X X

P23 Reverse

pressure

Be mindful of the causes of vacuum:

1. Sucking in suction catch pots when air

testing compressors

X X X

2. Draining vessels X X

3. Steaming out vessels X

4. Adding cold fluids to hot vessels (rapid

condensation)

X X

5. Internal reactions causing volume

shrinkage (e.g. polymerization or

rusting)

X X

6. Polythene sheets blowing over vents

and breather lines.

X X

(Continued)

291Appendix 2: Upset Conditions Table

Table A2.1 (Continued)Level


Con Det St Op

L1 More level Specify the design temperature of

equipment for the sum of hydraulic

head and vapor pressure (see P4).

X

L2 More level Locate vapor relief valves at the top of

the equipment so that they are not

‘drowned’ by liquid.

X

L3 More level Consider stressing pipelines and pipe

supports for the liquid full condition

(required for hydraulic pressure

testing).

X

L4 More level Consider the loading on foundations

and structures during hydrotesting. If

the vessel is totally flooded, design for

the worst case.

X

L5 More level Changes in interface level may result in

separate liquid phases passing forward

along the process route, should

protective systems be installed? (see

L7 and OT10).

X X X

L6 More level If equipment sizes are increased for any

reason consider the extra loading on

supports, particularly during

hydrotesting.

X X

L7 Less level The light phase will pass forward as

entrained fluid (see L5 and OT10).

X X X

L8 Less level Electric heaters or temperature probes

may be exposed. Low level trips

should be fitted to cut off the power

(see L11 and T5).

X X X

L9 No level Will the loss of level result in the loss of

liquid flow to a vital system such as

cooling water, lubricating oil or seal

oil? Should a protective system be

installed? (See OT10).

X X X

L10 No level Will the loss of level result in a gas

‘blow by’ from a high to a low

pressure system? Consider installing

flow chokes and protective system or

full flow pressure relief on the low

pressure system.

X X

(Continued)


Table A2.1 (Continued)Level


Con Det St Op

L11 No level Will the loss of level result in

overheating? Consider the effect of

loss of level in a boiler, a reboiler or

an electrically heated vessel. Install

low level trips (see L8 and T5).

X X X

L12 No level Install bunds round storage tanks sized for

1.1 times the storage tank capacity for

containment in the event of tank

rupture.

X

L13 Reverse level Consider splash filling vessels at a higher

elevation as opposed to filling under

liquid levels and possibly causing a

syphon effect. However, consider the

generation of static electricity if the

fluids are flammable.

X X X

L14 Reverse level Consider reverse level (i.e. from high to

low level) as a potential for reverse

flow.

X X X

Temperature


Con Det St Op

T1 More temperature Is the reaction exothermic? Can

the reactor ‘run away’? Consider

the need for protective systems

such as quenching, catalyst kill

or the equivalent.

X X

T2 More temperature Size the reactor cooling system

with excess capacity to prevent

a run away. Ensure that mixers

have a reliable power supply.

X

T3 More temperature Consider the potential for metal

fatigue due to temperature

cycling.

X X

T4 More temperature 1. Install low flow trips in fired

heaters

X X

2. Install high stack temperature

alarms in fired heaters

X X

3. Install high metal temperature

alarms in fired or electrically

powered heaters.

X X

(Continued)


Table A2.1 (Continued)Temperature


Con Det St Op

T5 More temperature 1. Install low flow trips in

electrically heated systems

X X

2. Install low level trips in

electrically heated vessels (see

L8 and L11).

X X

T6 More temperature Specify materials of construction

to give adequate allowance for

creep.

X X

T7 More temperature Inspect equipment for evidence of

creep on a regular basis. Note,

evidence of creep may manifest

itself suddenly after a number of

years of operation. Creep is a

cumulative effect � a number

of short deviations may lead to

serious damage in the future.

X

T8 More temperature Specify failure action of control

valves so as to minimize the

effect of failure.

X X X

T9 More temperature Consider the potential for

overheating when pumps or

compressors are blocked in.

X X X

T10 Less temperature Consider the effects of freezing in

cold environments (see C4).

Water can freeze in drain lines,

instrument trappings, relief

valves, valve bonnets, pump

casings, low point and fire water

lines. Examine the need for heat

tracing, thermal insulation

draining, maintaining a small

flow of fluids to maintain a

limited heat input.

X X X

T11 Less temperature Consider the possible effects of

crystallization of process fluids

in relief operations and

emergency drain/blow down

systems. Should these be heat

traced? (See C2)

X X X

(Continued)


Table A2.1 (Continued)Temperature


Con Det St Op

T12 Less temperature Consider the possible effects of

gels of high viscosity. Should

lines be heat traced? (see C3)

X X X

T13 Less temperature Specify the metallurgy such that

the metals will not enter a

brittle regime when depressured

(see P15).

X X

T14 Less temperature Specify the failure action of the

control valve so as to minimize

the effect of failure.

X X

Flow


Con Det St Op

F1 More flow Consider the effect of rise or fall of

levels in equipment.

X X X

F2 More flow Consider the potential for exciting

tube vibration in heat exchangers

and tube failure caused by fatigue

or wear from high velocity.

X X

F3 More flow Consider the potential for exciting

vibration in thermowells.

X X

F4 More flow 1. Install flow limiting devices X X X

2. Size relief systems for full flow

through the flow limiting device

X X X

3. Register the flow limiting device

as a protective system (see P9 and

OP2).

X X

F5 More flow Consider the potential for erosion in

bends due to solids or droplets of

liquids in gases.

X X

F6 More flow Install shallow bunded areas round

pumps, fired heaters and heat

exchangers to cater for spillage and

to retain foam blankets in fires.

X X

F7 No flow Install low flow trips on fired heaters

or electrically heated systems.

X X

F8 No flow Monitor flame arrestors for fouling

(see P12 and P21).

X X

(Continued)


Table A2.1 (Continued)Flow


Con Det St Op

F9 No flow Rod vents to prove they are clear (see

P12 and P21).

X X

F10 No flow Do not install temperature

measurement points in areas of no

flow.

X

F11 Reverse flow Install non-return valves in pumped

systems, in potential siphons, in

flexible loading/off-loading

systems.

X X X

F12 Reverse flow Can fluids be passed from one section

of the plant to another via drain or

vent/blow down systems?

Consider the potential hazards

from flow and mixing of

incompatible fluids.

X X X

F13 Reverse flow If a drain or vent system is choked,

can fluids pass from a high to a low

pressure vessel?

X X X

F14 Reverse flow Check the size of vent headers to

ensure that the pressure drop down

the header does not result in

overpressure or low pressure

equipment.

X X

F15 Reverse flow Can air be drawn into a hydrocarbon

system due to condensation,

process upset or flow regimes?

X X X

Concentration


Con Det St Op

C1 More

concentration

Consider what may happen if the

concentration of any reactant or

catalyst arises. Will the reactor

produce unwanted by-products or

become unstable? What warning

is needed?

X X X

C2 More

concentration

Can solids crystallize out of a liquid

phase? (see T11)

X X

C3 More

concentration

Can fluids become waxy or very

viscous? (see T12)

X X

(Continued)


Table A2.1 (Continued)Concentration


Con Det St Op

C4 More

concentration

Consider the effects of deposits on

instrument tappings, relief systems

and drain systems (see T10).

X X X

C5 More

concentration

Consider the effects of higher or

lower pH on metallurgy. It may

be prudent to assume that higher

concentrates may occur.

X X X

C6 More

concentration

Consider the possibility of build-up

in the concentration of impurities

in reactors, reboilers and

condensers. Should purge systems

be installed?

X X X

C7 More

concentration

Consider the possibility of erosion in

slurry systems. Should bends be

installed with extra wall thickness?

Should flushing points be fitted?

X X

C8 More

concentration

Consider the possible detrimental

effects of concentrated aqueous

spills or leakages into thermal

insulation. Could there be acid/

alkali/salt concentration which

will attack metal and cause stress

corrosion cracking?

X X

C9 More

concentration

Consider the possibility of

concentration of toxics or

unstable chemicals in the process,

e.g. acetylenes are particularly

unstable in high concentrates.

X X

C10 More

concentration

Consider the adverse reactions that

may take place if heat exchangers

leak. Could this affect the

process, the reactor chemistry or

the metallurgy?

X X

C11 More

concentration

Do control variables such as reflux

or reboil require resetting if

concentrations change?

X X X

C12 More

concentration

What are the maximum ground level

concentrations from vents? Are

they safe? Are they unpleasant/

offensive? Should the vents lead to

a flare for pyrolysis? Should the

stack height be increased?

X X

(Continued)


Other thanThis is a variable which requires the most careful consideration as it has so many dis-

guises. Some have already been addressed in other sections, but it should not be

assumed that all have been identified.

Table A2.1 (Continued)Concentration


Con Det St Op

C13 More

concentration

If the effluent concentration changes

can it create environmental

pollution?

X X X

C14 Less

concentration

Will the reaction stop if the

concentration of reactants or

catalyst falls? What warning is

needed?

X X

C15 Less

concentration

Do dilute concentrations create

excessive heat loads in separation

systems?

X X X

C16 More and less

concentration

Can the process enter a flammable

regime during normal or upset

operation? Consider startup/

shutdown and condensation.

X X X X

No Condition/Solution Most likely stage

Con Det St Op

OT1 Creep (see T7) X

OT2 Corrosion (see C5) X X

OT3 Erosion (see F5) X X

OT4 How will equipment age: do joints soften, harden or

crack? Will joints fail prematurely?

X X

OT5 Are minor components of the process incompatible

with process fluids, such as copper gaskets in

ammonia?

X X

OT6 Can fluids generate static charges under flow

conditions?

X X X

OT7 What by-products may be expected, e.g. X X X X

Pyrites X X X X

Polymers: unstable, explosive X X X X

Polymers: unstable, hydrolysis to toxic gases X X X X

(Continued)


(Continued)No Condition/Solution Most likely stage

Con Det St Op

OT8 Are contaminants (air and water) positive catalysts or

inhibitors?

X X X

OT9 Is there a risk of hydrogen blistering or other

unexpected corrosion effects?

X X

OT10 What happens if levels of liquid/liquid interface levels

are lost? (see L5, L7 and L9)

X X X X

OT11 Is air a potential ‘other than’ in the presence of

flammables?

X X X

Other than � failure

No Condition/solution Most likely stage

Con Det St Op

FA1 How does equipment fail?

1. Pump seals leak � is this tolerable? X X X

2. Heat exchangers corrode, erode, wear and fatigue X X

3. Vessels corrode and pit X

4. Structures rust (and corrode in acid environments

even under lagging)

X X X

5. Bearings or rotating equipment collapse � will this

create an intolerable seal leak?

X X X

FA2 What is the effect of instrument air failure, both local or

plant wide? Will the plant shut down safely?

X X X

FA3 What is the effect of service failure such as cooling water

or nitrogen purge?

X X X

Operations


Con Det St Op

OP1 What controls are imposed to prevent staff using unsafe

operational practices rather than operating procedures

e.g. audits, site tours, casual enquiries?

X X

OP2 What controls are imposed to prevent changes in design

intent? Are modification control procedures in place?

(see F4 and P9)

X X

OP3 What controls are in place to prevent the override of

trips/protective systems? Is the trip test system

‘operator resistant’?

X X X

OP4 What controls are in place to ensure that protective

systems e.g. relief valves and trips, are tested routinely?

X X

(Continued)


(Continued)Operations


Con Det St Op

OP5 What controls are in place to ensure that flow limiting

devices are not removed?

X X

OP6 Are operating instructions rewritten on a routine basis?

Are all operators aware of changes?

X X

OP7 Are maintenance procedures written and followed

correctly?

X X

OP8 Is there adequate communication between control

centers and outside operators?

X X X

OP9 Are audit procedures in place? X X


APPENDIX 3

Plant Separation Tables

Text and accompanying tables reproduced and adapted from “Process Plant Layout,”

edited by J.C. Mecklenburgh, Copyright IChemE 1985.

Detailed design requirements are to be found in the appropriate national codes, stan-

dards, and specifications for the individual classes of equipment and operation. The

following tables provide some guidelines of typical restraints which might be applied

during the early development of the site and plant layout with due consideration

given to safety.

Some of these typical quantities are taken from old codes of practice, etc. which

have now been superseded or withdrawn because the uncritical use of standard dis-

tances is no longer accepted as good layout practice. However, these standard distances

in the old publications can still be considered as good typical values for initial layout.

It must be emphasized, though, that no values in this appendix should appear in

the final layout without detailed checking that they apply to the circumstances of the

plant or site being designed.

During the final stages of preparing this book, I have been engaged to update

Mecklenburgh. I will revise the tables in my updated version to meet modern regula-

tory requirements. Until then, the following data is the best set in the public domain,

and is still a sound starting point.

301

Table A3.1 Site areas and sizes (preliminary)

Administration 10 m2 Per administration employee

Workshop 20 m2 Per workshop employee

Laboratory 20 m2 Per laboratory employee

Canteen 1 m2 Per dining space

3.5 m2 Per place including kitchen

and store

Medical Centre 0.1�0.15 m2,

minimum 10 m2Per employee depending on

complexity of service

Fire Station (housing 1 fire, 1 crash,

1 foam, 1 generator and 1

security vehicle)

500 m2 Per site

Garage (including maintenance) 100 m2 Per vehicle

Main perimeter roads 10 m Wide

Primary access roads 6 m Wide

Secondary access roads 3.5 m Wide

Pump access roads 3.0 m Wide

Pathways 1.2 m Wide up to 10 people/min.

2.0 m Wide over 10 people/min.

(e.g. near offices, canteens,

bus stops)

Stairways 1.0 m Wide including stringers

Landings (in direction of stairway) 1.0 m Wide including stringers

Platforms 1.0 m Wide including stringers

Road turning circles � 90 degree

turn and T-junctions

Radius equal to width of

road

Minimum railway curve 56 m Inside curve radius

Cooling towers per tower 0.04 m2/kW to

0.08 m2/kW

Mechanical draught

Natural draught

Boiler (excluding house) 0.002 m3/kW (height5 43 side)

302 Appendix 3: Plant Separation Tables

Table A3.2 Preliminary general spacings for plots and sites

Property

boundary

Controlroom

(non-pressurized)

Control

room

(pressurized)

Administrationbuilding

MainSubstation

Shippings,buildings,

warehouses

Loadingfacilities,

road,rail,water

Firepumphouse

CoolingTowers

ProcessFired

heaters

GasCompressors

Reactors

Highpressure

storage

spheres,bullets

Atm

osphericflam

mable

liquid

storagetanks

Aircoolers

Low

pressure

storage

spheres

ortanks,1bar

G

Plotlimits

Processcontrolstation

Processunitsubstation

Processequipment

(low

flashpoint)

Processequipment

(highflashpoint)

CryogenicO

2aplant

NA

30 NA

8 NA NA

8 8 8 NA

8 30 15 8 NA

8 15 NM 30 15 NA

30 60 30 60 60 60 NA

8 8 8 8 30 30 45 NA

30 30 30 30 60 30 45 30 7.5

30 30 30 75 60 75 60 60 30 7.5

30 30 30 60 60 60 60 60 30 15 2

60 30 30 60 60 60 60 60 30 15 10 2

CP 60 30 75 75 75 CP 75 30 CP 75 60 CP

CP 60 30 60 60 60 CP 60 30 CP 60 60 CP CP

60 30 30 60 60 60 60 60 30 15 7.5 5 60 60 NM

CP 60 30 75 60 60 CP 60 30 CP 60 60 CP CP 60 CP

60 60 60 60 60 60 45 60 30 NA NA NA CP CP NA CP 15

30 NA NA NA 30 NA 60 NA 15 15 15 15 60 60 15 60 NA NA

NM NM NA NA NM NA 45 NM 15 15 15 15 NM NM 15 NM NM NM NA

15 30 NM 60 60 60 60 60 30 30 7.5 5 CP CP 5 CP NA 15 15 2

15 30 NM 60 60 60 60 60 30 15 7.5 5 CP CP 5 CP NA 15 15 2 2

CP 30 NM 30 60 60 CP 45 30 CP 45 60 CP CP 30 CP CP 60 50 CP CP 30

Legend:NA Not applicable since no measurable distance can be determined.NM No minimum spacing established � use engineering judgment.CP Reference must be made to relevant Codes of Practice but see the following section on Preliminary Spacings for Tank Farm Layout.aAlso see the following section on Preliminary Spacings for Tank Farm Layout for minimum clearances.

Notes:

(a) Flare spacing should be based on heat intensity with a minimum space of 60 m

from equipment containing hydrocarbons

(b) The minimum spacings can be down to one-quarter these typical spacings when

properly assessed.

Table A3.3 Preliminary access requirements at equipmentAccess Item of equipment

Permanent

Ladder

1. Gate and globe valves � DN 80 (in mm) and smaller at vessels when

located 3.5 m above grade

2. Check valves � all sizes at vessels when located 3.5 m above grade

3. Gauge glass 2 m above access surface, or inaccessible by portable ladder

or platforms

4. Pressure instrument on vessels 2 m above access surface, or inaccessible

by portable ladder or platform

5. Temperature instrument on vessels 2 m above access surface, or

inaccessible by portable ladder or platform

6. Handholds located 3.5 m above grade

Items located over platform

Platforms 7. Manholes

8. Heat Exchange Units

9. Process blinds

10. Relief valves on vertical vessels DN100 and larger

11. Control valves � all sizes

12. Cleanout points

Items located adjacent to platform

13. Gate and globe valves � DN100 and larger at vessels

14. Motor operated valves

15. Relief valves � DN80 and smaller

16. Relief valves on horizontal vessels DN 100 and larger

17. Level controls an gauge glass on vessels

18. Sampling valves on vessels


Table A3.4 Preliminary minimum clearances at equipmentItem Description Clearance (m)

Roads 1. Headroom for primary access roads or major maintenance vehicles

2. Width of primary access roads

3. Headroom for secondary roads and pump access roads

4. Width of secondary roads and pump access roads

6.0

6.0

3.0�4.5

3.0�4.5

Railways 5. Headroom over through railways from top of rail

6. Headroom over dead-ends and sidings from top of rail

7. Clearance from track centerline to obstructions

6.7

5.1

2.4

Access, walkways and

maintenance

clearances

8. Headroom over platforms, walkways, access ways, maintenance areas

9. Width of stairways, back to back of stringers

10. Width of landings in direction of stairway

11. Width of walkways at grade or elevated

12. Vertical rise of stairways � one flight

13. Vertical rise of ladders � single run

14. Clearance under furnace burner nozzles for maintenance purposes

2.5

0.75

0.9

0.75

4.5

7.5

2.1

Platforms 15. Towers,

16. Vertical

17. and

18. horizontal

19. vessels

Distance of platform below bottom of manhole flange (side platform)

Width of manhole platforms from manhole cover to outside edge of platform

Platform extension beyond centerline of manhole flange (side platform)

Distance of platform below underside of flange (top platform)

Width of platform from three sides of the manhole (top platform)

0.3

0.75

0.75

0.2

0.75

20. Horizontal

21. exchangers

Clearance in front of channel or bonnet flange

Clearance from edge of flanges

1.2

0.3

22. Vertical

23. exchanger

Distance of platform below top flange of channel or bonnet

Width of platform from three sides of flange

1.5 max

0.6

24. Furnaces Width of platform at sides of horizontal and vertical tube furnace 0.75

25. Width of platform at ends of horizontal tube furnaces 1.0

Pipeways 26. Pipeways not crossing roads 3.0

Table A3.5 Handling facilities for equipmentItem Equipment and equipment part handled Handling facility

Vertical vessels 1. Manhole covers (up to DN 600) and vessel

trays

Davits

2. Bottom manholes Hinged

3. Internals of fixed bed reactors, catalyst, tower

packings etc.

None

Horizontal exchangers

(at grade or in

structure)

4. Removable tube bundles, and other removable

parts except exchanger shells, shell covers,

and floating head covers

Pulling beams or posts, for moving the bundle

within the shell. Trolley beams for groups

requiring up to four such beams. Trolley beams

shall be provided with either (a) two trolleys,

one capable of handling the entire load and the

other half-capacity, or (b two half-capacity

trolleys

5. Exchanger shells None

6. Fixed tube sheet exchangers None

Shell covers and floating head covers Shell davits or overhead hitching points

Vertical exchangers 7. Stationary tube

sheet at lower end

Tube bundles, channels,

and channel covers

Hitching points

8. Shell covers and

floating head covers

Jib crane, davit, or hitching point

9. Entire small-size units Hitching point or trolley beam

10. Stationery

tube sheet

at upper

end

Units

designed

for

removing

tube

bundle

from shell

Tube bundles,

channels and

channel

covers

Trolley beam

11. Shell covers and

floating head

covers

Hitching points

12. Entire small-

sized units

Hitching point

13. Units designed for removing

the shell from the bundle:

the entire unit or any of its

component parts

Hitching point

14. Fixed tube sheet exchangers Shell davits or hitching points

Pumps, compressors and

drivers (housed or

otherwise inaccessible)

15. 100 kg�2t

incl.

Parts of horizontal centrifugal

pumps and steam drivers

Overhead hitching point or trolley beam

16. Cylinder heads and pistons

only of reciprocating pumps

and horizontal reciprocating

compressors

17. Over 2t Parts of centrifugal pumps,

compressors and steam

drivers including top halves

of compressors, and turbine

covers

Trolley beam or overhead travelling cable

18. Cylinder heads and pistons

only of reciprocating

compressors

19. Power cylinders only of

inclined type reciprocating

compressors

20. Parts of vertical-type pumps and drivers Overhead hitching point

21. Electric motors and rotors None

Piping (housed or

otherwise inaccessible)

22. Relief valves, DN 1003 150 and larger Hitching points or davits

23. Blanks, blind flanges, fittings, and valves other

than listed above and weighing more than

150 kg

Hitching points or davits when subject to

frequent removal for operation or maintenance

PRELIMINARY SPACINGS FOR TANK FARM LAYOUT

1. Where space allows, greater distances than these should be used. The incorporation of

these minimum distances into a design can only be made after a proper assessment.

2. Flammable liquids for this table are defined as those with flash points up to 66�c.3. Measured in plan from the nearest point of the vessel, or from associated fittings

from which an escape can occur when these are located away from the vessel.

4. A group of vessels should not exceed 10000 m3 unless a single vessel. Spacing between

such groups should be a minimum of 15 m between adjacent vessels. The bund to

have a net volume not less than 10% of the capacity of the largest tank in the bund

after deducting volume up to bund height of all other tanks in the same bund.

5. If this distance cannot be achieved, the need for suitable fire protection of the

cable or pipeline should be considered.

6. For bunded tanks containing water-soluble non-hydrocarbons, power cables and

pipelines at ground level should be outside the bund and so protected by the fund

from fire in the tanks.

7. Measured in plant from the nearest part of the bund wall except where otherwise

indicated.

8. A group of tanks should not exceed 60000 m3. Spacing of the nearest tanks in any

two such groups, which may have a common bund wall, should be such that the

tank in one group should be a minimum of 15 m from the inside top of the bund

of any adjacent group(s).

9. The zone may be beveled across its upper corners providing all parts of the vessel

more than 3 m from the zone edge.

Table A3.6 Preliminary minimum distances (note 1) for liquefied oxygen (5,6)Distance (m)

To site boundary 30

To site roads 15

To process units and buildings containing combustible materials and ignition

sources

30

To outside fixed combustible materials 5

To buildings containing flammable fluids 45

To road and rail loading areas 15

To overhead power lines and pipebridges 30

To other above-ground cables and important pipelines or pipelines containing

flammables

15

To underground cables, trenches 10

To low-pressure gas storage 30

To compressed gas storage: flammable 30

non-flammable 15

To liquefied pressure and refrigerated storage: flammable 45

non-flammable 15

To liquid storage tanks: flammable (note 2) 45

non-flammable (note 2) 30


Table A3.7 Preliminary minimum distances (note 1) for liquefied, flammable gasesItem Material stored

Hydrocarbons Non-hydrocarbonsinsoluble inwater

Non-hydrocarbonssoluble inwater

Pressure storage (Notes 3,4)

To boundary, process units, buildings

containing a source of ignition, or

any other fixed sources of ignition,

e.g. process heaters

For example:

Ethylene

60 m

C3 45 m

C4 30 m

For example:

Methyl

Chloride 23 m

Vinyl

Chloride 23 m

Methyl-vinyl

ether 23 m

Ethyl Chloride

15 m

For example:

Methylamines

15 m

To building containing flammable

materials e.g. filling shed

15 m 15 m 15 m

To road or rail tank wagon filling points 15 m 15 m 15 m

To overhead power lines and

pipebridges

15 m 15 m 15 m

To other above-ground power cables

and important pipelines or pipelines

likely to increase the hazard

(Note 5)

7.5 m

(Note 5) 7.5 m See Note 6

Between pressure storage vessels One-quarter of sum of diameters of adjacent tanks

but not less than 1.8 m for # 50 m3 or less than

15 m for 750 m3

To low pressure refrigerated tanks 15 m from the bund wall of the low pressure tank,

but not less than 30 m rom the low pressure

tank shell

To flammable liquid (note 2) storage

tanks

15 m from the bund wall of the flammable liquid tank

To liquid oxygen storage As defined above under ‘Liquefied Oxygen’

Zone 1 extent 1 m sphere around relief valve discharge

Zone 2 horizontal extent from edge of

tank

For example:

Ethylene

30 m

C3’s 30 m

C4’s 20 m

For example:

Methyl

Chloride 15 m

Vinyl

Chloride 15 m

Methyl-vinyl

ether 15 m

Ethyl Chloride

10 m

For example:

Methylamines

10 m

Zone 2 height of zone 2603 relief diameter above relief valve discharge

(see note 9)

(Continued)

309Appendix 3: Plant Separation Tables

Table A3.7 (Continued)Item Material stored

Hydrocarbons Non-hydrocarbonsinsoluble inwater

Non-hydrocarbonssoluble inwater

Low pressure refrigerated storage (Notes 7,8)

To boundary, process units, buildings

containing a source of ignition, or

any other fixed sources of ignition

For example:

Ethylene

90 m

C3’s 45 m

C4’s 15 m

For example:

Ethylene

oxide 15 m

To building containing flammable

materials e.g. filling shed

15 m 15 m

To road or rail tanker filling point 15 m 15 m

To overhead power lines and

pipebridges

15 m 15 m

Between low pressure refrigerated

tanks

One-half of sum of diameters of adjacent tanks

To flammable liquid (note 2) storage

tanks

Not less than 30 m between low pressure

refrigerated LFG and flammable liquid tank

shells, but LFG and flammable liquids must be

in separate bunds

To pressure storage vessels As defined above under ‘Pressure Storage’

To liquid oxygen storage As defined above under ‘Liquefied Oxygen’

Zones 1 and 2 As defined above under ‘Pressure Storage’


Table A3.8 Liquids stored at ambient temperature and pressurePreliminary minimum clearance

Dim

(Fig A3.1

& A3.2)

Diameter of

tank

Water and non-

flammable

liquids

Class A and B Products Class A and B products (flash

point ,32�c)Class C products

Fixed roof Floating roof

A Up to 6 m � 3 m 3 m

6�30 m Half tank dia. Half tank dia.

Over 30 m 15 m 6 m Half tank dia.

B All 1.5 m Least of:

Half dia. of largest tank, dia. of smallest

tank, 15 m. (min 6 m)

Least of:Half dia. of largest

tank, 6 m

Half dia. Smallest

tank Min 3 m

C All � Dia. of largest tank Min 10 m Dia. of largest tank (Min 6 m) Dia. Of largest tank

Min 6 m

D All 6 m 15 m 6 m 6 m

E All � 15 m 6 m 6 m

F All 6 m 7.5 m 7.5 m 7.5 m

G All 5 m 30 m 30 m 15 m

H All Depends on

building lines

30 m 30 m 15 m

J All � 30 m 30 m 15 m

K All � 15 m 15 m 15 m

M All � 15 m 15 m 15 m

N All 7.5 m 7.5 m 7.5 m

P All Outside bund Outside bund Outside bund Outside bund

Q All � Bund Width Bund Width Bund Width

U Up to 3.5 m � 1.5 m 3 m �V Over 3.5 m � 3 m 3 m �W All � All bund to wall height All bund to wall height �X Up to 3.5 m � 5 m �

3.5�5 m � 6 m 15 m �Over 5 m � 15 m �

Y Up to 3.5 m � 2 m �3.5�5 m � 2.5 m �Over 5 m � 5 m 5 m �

Z All � 5 m 5 m �Max capacity/bund � 60000 m3 120000 m3 �The spacing and arrangement of tankage can vary with each application (note 1).Class A products have closed flash points below 23�c.Class B products have closed flash points between 23�66�c.Class c products have closed flash points above 66�c.

FD

E

Boundary tence

Roadway

(Un)loading Q

K

H

B C

Y X

J

M,N,P

A

G

Pumps

Pipe track

Heater

Building

Figure A3.1 Preliminary tank farm layout (A) plan view.

Serviceway

D

Road

Roa

dway

F

1.5 m

1m

1½

1Z

W

Tankvent

U

V

Acessway

Figure A3.2 Preliminary tank farm layout (B) elevation.


PRELIMINARY ELECTRICAL AREA CLASSIFICATION DISTANCES

Note that these are for preliminary layout only in well-ventilated locations.

DefinitionsLiquid5 fluid below atm. b.p. (see Table A3.6 for definitions of Class A, B and C

fluids)

Gas5 fluid above atm. b.p.

Legend Distance

A from outside of tank to outside of bund at top

B between any 2 tanks in one tank bund

C between any two tanks in adjacent bunds

D from tanks to main plant roads

E between tanks and buildings containing flammable material

F from toe of bund to center line of any plant roads

G from tank to center of railway

H from tank to boundary fence

J between tank and fired heaters or ignition sources

K from tank to road or rail filling

M from tank to ground underneath power lines and pipe bridges

N from tank to power cables or pipelines

P from tank to ground above buried cables or pipes

Q from tank to combustible materials

U from tank vent to top of zone 1

V from outside of tank to edge of zone 1 and zone 2

W from tank rim to junction of zone 1 and zone 2

X from outside of tank to edge of zone 2

Y from center line of bund wall to edge of zone 2

Z from ground to top of zone 2

3m

3m

3m

x diameter

Pump seal

Figure A3.3 Preliminary extent of zone 2 around a pump seal.


Table A3.9 Electrical area classification distances for centrifugal pumpsSeal Fluid conditions Zone 1 Zone 2 X in

Fig A3.3

Any (inc. reciprocating pumps) Liquid , atm. b.p.$ambient temperature

None Diameter of

pool1 6 m

Mechanical seal, external

throttle bush, drain, atm.

b.p. . ambient temperature

No bush need be used for cases

marked � if X doubled

Class A, . atm. b.p.

temp ,100�ctemp ,200�ctemp .200�cClass B, . atm. b.p.

temp ,200�ctemp .200�cClass C, . atm. b.p.

temp ,250�ctemp .250�c

0.3 m

sphere

around

seal

20�

40

60

20�

50

As liquid

20

Mechanical seal, external

throttle bush, vent to stack,

atm. b.p. # ambient

temperature

Liquefied C4’s (i.e. atm.

b.p.� 0�c)Liquefied C3’s and

lighter HC (i.e. atm.

b.p.� 20�c) (see notebelow)

Liquefied non-

hydrocarbons

0.3 m

sphere

around

seal

40

60

20�30

Note: Zone 1 for C3’s which may be up to 3 m depending on seal performance.

Table A3.10 Electrical area classification distances for equipment other than pumpsItem Condition Zone 1 Zone 2

Compressors in open-

sided houses

Gases See note below See Fig A3.4

Note: Zone 1 is 0.5 m around any gland, seal, drain parts, vents except 1 m is allowed around

a seal oil lid and vent or a seal oil trap

Equipment in normal

buildings

Outdoor distances as shown in Fig A3.5

Joints and flanges on

pipes, fittings and

process equipment

Liquid

Gas lighter than

air

Gas heavier than

air

None

None

None

X5 diameter of

pool1 6 m in Fig

A3.3

3 m horizontal status,

7.5 m above, 5 m

below

7.5 m horizontal

radius, 5 m above and

down to floor

Note: Valve glands can be treated as pump seals

(Continued)


Table A3.10 (Continued)Item Condition Zone 1 Zone 2

Relief valves, vents etc. High velocity, gas

lighter than air

High velocity, gas

heavier than air

1 m sphere

1 m sphere

See Fig A3.6

H5 100, R5 60

See Fig A3.6

H5 260, R5 120

Low velocity,

frequent release

Low velocity,

infrequent release

1.5 m sphere

None

3 m sphere

Sample points ,6 mm

diameter

Liquids near

ambient

temperature,

into open

None See Fig A3.7

Other liquids into

closed system

None 15 m radius, 3 m up,

down to floor

Gases into closed

system

None See ‘Joints, and

flanges on pipes

etc.’ above

Process water drain

point into open, at

grade used regularly

Liquids See note below X5 diameter of

pool1 6 m in Fig

A3.3

C3 under pressure See note below 3 m high3 45 m

radius

C4 under pressure 3 m high3 30 m

radius

Other gases under

pressure

3 m high3 20 m

radius

Note: Zone 1 is a cylinder 1 m radius and 1.5 m high for liquids and 5 m radius and 1.5 m

high for gases

Instruments etc., near or

at grade

Liquids See note below X5 diameter of

pool1 6 m in

Fig A3.3

Gases See note below Flanges as pipe joints

Drains as sample

points

Note: Zone 1 is not needed for infrequent spills but otherwise is a cylinder 3 m high by

radius of 3 m if below atm. b. p. and 5 m if above atm. b. p.

Road or rail (un)

loading

Liquids

Gases

See Fig A3.8 for zones 1 and 2; H5 1 m

See Fig A3.8 for zones 1 and 2; H5 3 m

Ship (un)loading 20 m

around3Nhigh

None

(Continued)


Table A3.10 (Continued)Item Condition Zone 1 Zone 2

Unloading only None 20 m around except

seaside3 10 m

high

Fixed roof tank Liquids See Fig A3.9 for zones 0�2

See also Preliminary Spacings for Tank

Farm Layout

Floating roof tank Liquids See Fig A3.10 for zones 0�2


Farm Layout

Pressure storage vessel Gases See ‘Joints and flanges on pipes, etc.’,

‘Relief valves’, ‘Process water drain

point’, as appropriate

Low pressure

refrigerated tank


Farm Layout

Open topped oil water

separator

Liquids See Fig A3.11 for zones 0�2

Open topped drains and

effluent pits

Liquids See Fig A3.11 for zones 1�2

Drums in open Liquids See Fig A3.12

(only if being

filled)

3 m around drum

area

X

H

Im

Im

R

R

Z

Source of potential leakZone I

Zone 2Xm Outdoor

distance

Dimension Heavierthan air

Lighterthan air

Caveat

Height

Distance

H

X

Radius R

Height Z

3m

15m 5m

5m3m

3m

Sourceheight + 7.5m

Sourceheight + 7.5m

Wallopening

Distanceto wall

Figure A3.4 Preliminary extent of zone 2 in compressor house.


Dimension Heavierthan air

Lighterthan air Caveat

Height H

Diatance X

Radius R

Height Z

3m

15m

3m

3m

5m

5m

Sourceheight + 7.5m

Sourceheight + 7.5m

Wallopening

Distanceto wall

Self-closing door orAir lockdoor

Source of potential leakZone I

Zone 2

Xm Outdoordistance

Normaldoor

Louvres Louvres

X

X

Normaldoor

Normaldoor

FanFan

X

Source

X

Figure A3.5 Preliminary extension of zones to outside building.


0.5m

1.5m

Sample point(16mm bore)

Pool diameter

Figure A3.7 Preliminary extent of zone 2 around a liquid sample point.

Nozzle position

Rx nozzle digm.

Hx nozzle digm.

5m

Figure A3.6 Preliminary extent of zone 2 around a relief valve, etc.


Zone I

Zone 2

Extra zone 2with canopy

1.5m

1.5m

1.5m

1.5m

1.5m

7.5m 7.5m min

4.5m min

4.5mmin.

2m

Hm

Spillage drain

7.5mmin.

1.5m

Spillage drain

(A)

(A)

(B)

Figure A3.8 Preliminary extent of zones 1 and 2 for road or rail (un)loading areas: (A) elevation; (B)plan view.


Tank vents U

U

2m

2m

U

U

Liquid surface

5m

X

Y

Tank vents

Tank diam. U X Y

Liquid surface

Bund wall

Zone 0

Zone 1

Zone 2

Up to 3.5 m3.5 to 5 mOver 5 m

1.5 m2.5 m

15 m3 m3 m

5 m 2 m

5 m6 m

(A)

(B)

Figure A3.9 Preliminary extent of zones 0, 1, and 2 for a fixed roof tank: (A) double-walled tank;(B) single-walled tank.


1m

3m

7.5m 7.5m

3m

3m

Liquid surface

Zone 0

Zone 1

Zone 2

(A)

(B)

Figure A3.11 Preliminary extent of zones 0, 1, and 2 in open-topped constructions: (A) open-topped oil/water separator; (B) quench drain channel or effluent interceptor pit.

3m

3m

5m

Floating roof

Bund wall

Zone 1

Zone 2

Seat

5m

Note.This type of tank is not usually used to store liquidswith flash points greater than 32°C

15m

Figure A3.10 Preliminary extent of zones 1 and 2 for a floating roof tank.

2.5 m

0.5 m

1m

Note: Drum areas withoutfilling are zone 2

Figure A3.12 Preliminary extent of zone 1 for drum filling in open.


SIZE OF STORAGE PILES

1. The height (h, in m) of a right conical pile is given by:

h53V tan2O

π

� �1=3

where V5 volume (m3) and O5 angle of repose (commonly 37�). If the conveyorangle is φ, the horizontal length of the conveyor (L1 in m) is L15 h cot φ. Theangle φ is commonly 18�. The radius (r, in m) of the bottom of the pile is

r5 h cot O. It follows that the minimum length (L2 in m) required for a conveyor

and pile in one straight line on plan is:

L2 5L1 1 r5 hðcot φ1 cot OÞ2. Approximate volume (V, in m3) of a straight conical pile is:

V 5 h2L3 cot O1π3h3cot2O

Where L3 (in m) is the length of the top of the pile.

3. Approximate volume (V, in m3) of a curved conical pile is:

V 5 h2Rα cot θ1π3h3cot2θ

Where R5 radius of curve (in m) and α5 size of arc in radians

4. Approximate volume of closed warehouse (V, in m3) is:

V 5 h2L4 cot θ

Where L4 (in m) is the length of the pile. This equation assumes fully triangular

cross-section and no spaces around the piles for conveyors or mechanical unload-

ing equipment. Thus the equation can be used as it is for underground conveying,

but for unloading from one side, add 5 m to the width of the store. Also add

10�20% to the length to allow for dead spaces.


APPENDIX 4

Checklists for Engineering FlowDiagrams

This appendix is reproduced and adapted from Sandler, H.J. and Luckiewicz, E.T.

(1987) Practical Process Engineering: AWorking Approach to Plant Design.

The placement of the required equipment and necessary connecting piping with iden-

tifications on an engineering flow diagram (such as a PFD or P&ID) is only the initial

phase of the work required to make the diagram complete. A great many details must

be added so that the process being described on the diagram fulfils its function in an

economical and safe manner and is in compliance with environmental requirements.

While the details entered vary considerably from project to project, many items are

common to most projects. The following checklists regarding major components

found on many engineering diagrams should be kept in mind while preparing the dia-

grams. The lists concern the appurtenances on equipment and piping and also review

primary instrumentation as well as important safety considerations.

Although the lists are not fully comprehensive, the items in them relate to situa-

tions most likely to be encountered and alert the process engineer to the type of

details to be considered for inclusion on engineering flow diagrams.

Short elaborations of the various items in the checklists follow to give guidance

for their representation on an engineering flow diagram.

323

Table A4.1 Checklists for engineering flow diagramsType ofequipment

Item Notes

Vessels Nozzle types Connections:

• Couplings, generally for connections of 32mm

nominal bore (NB) diameter or smaller.

• Flanged nozzles, for any size of connection.

Nozzles with a diameter smaller than 50mm NB

on low-pressure vessels are fragile, and

connections should be made by means of

reducing fittings or flanges.

• Non-metallic connections are often of limited

size. Manufacturer catalogues should be consulted.

Vents • Closed vessels are generally vented.

• Vents with only air or low amounts of water

vapor terminate above the top of the vessel.

• Vents with vapors that may be harmful but are

not toxic or lethal (i.e. hot gases or hot vapors)

may terminate outdoors above adjacent structures

in accordance with pertinent regulations.

• Dangerous vapors or gases pass to a flare system

or to a collection system for further treatment.

• An invert or ‘rain hat’ protects vessel contents

from precipitation.

• The diameter of the vent line is generally made

equal at least to that of the largest liquid line

entering the vessel.

Drains • Most vessels are provided with drains.

• Drain connections can originate from a discharge

line or from the vessel itself.

• The destination of a drain should be indicated.

Vortex breakers • A vessel with a low liquid level may require a vortex

breaker to prevent gases from entering a pump

suction.

Overflow pipes • Open or low-pressure vessels handling liquids

require an overflow connection.

• The top invert of the overflow nozzle must be

sufficiently below the top tangent line to

accommodate the constriction head at the

maximum flow rate through the nozzle.

• Overflow lines for closed, blanketed vessels or

those under a slight negative pressure must be

sealed in a suitable liquid loop seal or by means

of a mechanical sealing trap.

• Seal-leg heights should be included on the

engineering flow diagram if required to prevent

over pressuring or collapse of the vessel.

• A high-level switch can be used to close valves in

inlet lines or to stop supply pumps.

(Continued)

324 Appendix 4: Checklists for Engineering Flow Diagrams

Table A4.1 (Continued)Type ofequipment

Item Notes

Sample

connections

• Sample connections may be placed on a vessel,

on a discharge line at the bottom of the vessel, or

on a continuously flowing line from a pump

taking suction from the vessel.

• Samples may be taken from the top of a vessel

through a suitable sampling mechanism.

• A series of connections is required for vessels

which may have stratified layers.

Dip legs • Dip legs are required to diminish the generation

of static-electricity effects with flammable liquids,

reduce foaming, introduce reactants into an

agitated vessel, return heated, viscous fluids to

the suction nozzle of a circulating pump, create a

barometric leg in a vacuum system, or prevent

corrosive effects due to aeration.

• A weep hole or a siphon break is frequently

required in dip legs other than those acting as

barometric legs.

• Weep holes in lines with vapor-liquid flow

should be of minimum size and located in the

neck of the nozzle.

Gooseneck inlets • A gooseneck inlet is an alternative to a dip leg in

a metallic vessel to reduce static-electricity

effects.

• A weep hole is not required with a gooseneck

inlet.

Baffles • Baffles are frequently required to improve

agitation.

• The vendor of agitation equipment usually

recommends the baffle sizing and configuration

best suited for the respective vessel dimensions.

Manholes • Manholes are required for inspections, cleanout

and maintenance.

• For ventilation and as an escape or rescue port,

one manhole should be in the head or top of a

vessel with another on the side or bottom.

• Manholes are normally of 600mm diameter but

never smaller than a 400mm3 600mm ellipsoid

for use in special instances.

• Unwanted dead space in a manhole can be

avoided by use of an internal ‘hat’ or a curved

flush cover.

• Number, sizes and special configurations are to

be shown on engineering flow diagrams.

(Continued)

325Appendix 4: Checklists for Engineering Flow Diagrams


Item Notes

Handholds • Handholds are used on small vessels for

inspecting limited areas, feeling for the integrity

of a vessel, and charging or removing catalysts,

desiccants, or packing from columns or vessels.

• Number, sizes and special configurations are to

be called out on flow diagrams.

Air locks • An air lock is a small vessel mounted at the top

or directly beneath a bin or reactor to introduce

or remove solids or liquid intermittently.

• Suitable valves are placed upstream and

downstream on the air lock. One or both valves

are always closed to isolate the bin or reactor

from its surroundings.

• An inert purge may be used to preclude air or

water vapor from a bin or reactor or to prevent

the atmosphere in the vessel from escaping to the

surroundings.

Rotary feeders • A rotary or star feeder permits a continuous,

controlled, adjustable flow of solids into or from

a vessel while isolating it from its surroundings.

• An inert purge may be used to preclude air or water

vapor from a vessel or to prevent the atmosphere in

the vessel from escaping to the surroundings.

Live bottoms • The containing vessel should have a coned

bottom whose angle is greater than the angle of

repose of the solids.

• An internal pair of small cones, one inverted

from the other, facilitates the outflow of solids.

• An air lance is frequently used to prevent or

disrupt bridging.

• One or more mechanical vibrators attached to

the outside of the cone bottom helps to induce

the flow of solids.

• A ‘live bottom’, or pulsating section, is often

inserted between the cone section and the

discharge valve for difficult solids-flow problems.

Vessel jackets • Carbon steel vessel jackets offer a low-cost means of

transferring heat to glass-lined or high-alloy vessels.

• Liquid media enter the bottom of a jacket and

follow a tortuous path through the baffled or

dimpled annulus to exit at the top.

• Condensing media enter at the top, and the

condensate leaves at the bottom; baffling is not

needed. A vent should be provided.

(Continued)



Item Notes

• Steam and water may be used alternately, but

vents and drains must be provided in utility

piping so that the two media do not contact one

another. Controls should be provided in utility

piping to glass-lined tanks to prevent high

temperature differentials between the vessel and

the jacket contents.

Heat transfer

panels

• The panels are used to transfer modest amounts

of heat or in retrofitting a vessel. They are

frequently used to maintain the temperature of

vessel contents against heat loss or gain.

• External panels may be constructed of carbon

steel or galvanized sheet regardless of the vessel

material.

• They are available in a variety of shapes to fit

various contours. They may be supplied with

different internal paths, serpentine configurations,

and positions for inlet and discharge connections.

• Usually they are given item numbers separate

from that of the vessel.

Internal coils • These coils are placed inside agitated vessels for

improved heat transfer. They may be used by

themselves or in addition to a jacket.

• They may consist of more than one concentric

set of coils provided there is adequate space for

circulation between coils and rows.

• The materials of construction must be

compatible with the contents of the vessel

at the extremes of temperature expected within

the coil.

Vessel tracing • Heat needed to hold a storage vessel at a required

temperature or to provide freeze protection is

sometimes supplied by tracing the vessel with

electrical tape. Heat transfer is enhanced by the

use of a special grout or mastic.

• Sheets of metallic foil can be used to cover

vessels constructed of plastic, resinous, or other

non-metallic materials before the application of

electric-tracing tape to prevent the generation of

localized hot spots.

• The presence of electric tracing on a vessel is

normally not indicated in the body of an

engineering flow diagram but is noted with the

item number, name, and description.

(Continued)



Item Notes

Insulation • Some design groups indicate insulation by

portraying a section of it on a vessel in the body

of the flow diagram; others include a notation

‘Insulate’ or the purpose of the insulation in the

section with the item description.

Removable spool

pieces

• Vessels which personnel can enter should be

provided with removable spool pieces at liquid

inlet nozzles servicing the vessel.

• Spool pieces are removed and piping or valves

blanked off whenever a person is in the vessel.

Vessel position • Engineering flow diagrams should show whether

vessels are in a vertical, horizontal or inclined

position.

Vessel supports • Representations of vessel supports should be

included in flow diagrams.

Agitators or

Mixers

Location of

agitators

• Most agitators enter at the top, and the shaft

extends straight down or at an angle.

• A side-entering agitator is mounted through a

nozzle below the liquid level.

• In some instances, the shaft may enter through

the bottom head.

Types of agitators • The type of agitator (propeller, turbine, anchor

etc.) should be depicted on the flow diagram.

• Multiple sets of blades are sometimes required

and should be shown.

• Auxiliary internals such as vessel baffles or draft

tubes are often associated with agitators.

• In the absence of prior similar or pilot-plant

experience, a reputable agitator or mixer

manufacturer should be consulted to determine

the type of agitator or auxiliaries.

Steady bearings • A steady bearing should be denoted on a flow

diagram if one is included to reduce the size of

the shaft diameter.

Seals • Packing or a mechanical seal is used to prevent

leakage of air into or vapor out of a closed vessel.

• Shaft packing or a mechanical seal is required for

a side- or bottom-entering agitator.

• While shaft packing or mechanical seals generally

are not noted on engineering flow diagrams,

special lubrication or cooling systems are

depicted.

(Continued)



Item Notes

Pumps Types of pumps • A large variety of pumps is available for use in a

processing plant (e.g. centrifugal ump, reciprocal

pump, gear pump, metering pump, diaphragm

pump (air-operated), in-line pump).

Types of drivers • Since most pumps are driven by electric motors,

many design groups usually omit a driver symbol

if the driver is an electric motor.

• An air motor may be employed if the power

requirement is low or for safety considerations in

electrically hazardous areas.

• Hydraulic motors are used for high-torque

requirements in special applications.

• Steam turbines are used as an economy measure

when exhaust steam is available. They are also

used for critical pumps or compressors when

flow is to be maintained during a power failure.

• Diesel and gasoline engines are used as

alternatives to steam for critical services. They

are also used as prime movers in remote

locations.

Valved vents and

drains

• Vents and drains can be provided on a pump

casing rather than on the discharge or suction

piping.

• The type and size of valve are to be shown at the

appropriate location on the pump symbol.

• T-bar handles can be welded to casing plugs.

They should be indicated on the flow diagram.

Quench system • Pump packings or seals must be protected from

high-temperature fluids by a suitable cooling-

water quench.

• Some types of seals require special jacketing.

• Water piping to and from pump quench or seal

jackets is shown on the flow diagram.

Flushing and seal

fluid systems

• Water or other compatible fluid is sometimes

required to flush slurries, crystallizing solutions,

or corrosive liquids from shaft packings.

• Liquids may be needed to flush a mechanical seal

to remove frictional heat or to prevent a slurry

from reaching a seal face.

Jacketed pumps • Jacketed casings are used to maintain the fluid in

the pump at an elevated or a chilled temperature.

• Jacketing is also used to protect the pump casing

from excessive temperatures.

(Continued)



Item Notes

Tracing and

insulation

• Heat tracing may be required to prevent residual

fluid in a pump from freezing when the pump is

not operating.

• Insulation may be required over tracing or

jackets to conserve heat or energy. It may also

be needed for personnel protection or antisweat

purposes.

• Tracing and insulation requirements are included

with the equipment description.

Pump recycle

lines

• A continuous recycle line is sometimes installed

to maintain minimum flow through a pump.

• A restriction orifice is normally placed in the

recycle line.

Heat

Exchangers

Types and

configurations

• The representation of the heat exchanger on the

flow diagram should reflect as much as possible

its particular type and configuration.

• The location of the nozzles should approximate

those on the exchanger as it is to be placed in

service.

Sloping

exchangers

• Single-tube-pass horizontal exchangers in

condensing service are frequently sloped in the

direction of the condensate outlet.

Position of

nozzles

• Liquids being heated should enter exchangers at

the low point and leave at a high point to

prevent buildup of air or other gases which may

come out of solution.

• If suspended solids are present, the flow should

be from the top down to prevent a buildup of

solids.

• Gases may be removed from a two-shell-pass

exchanger by an external restricted vent or from

the channel of a vertical U-tube exchanger by an

internal weep hole. The orifices are sized to

preclude excessive bypassing of fluid.

Backflushing

exchangers

• Periodic backflushing is needed if a fluid

contains suspended matter and there is a wide

variation in its velocity as it passes through the

exchanger.

• Cross piping and valving are provided to

direct normal inlet fluid to the discharge

nozzle while effluent leaves the inlet nozzle

carrying dislodged suspended material to the

discharge piping.

(Continued)



Item Notes

Impingement

protection

• In many exchangers, it is necessary to protect the

tubes or internals from erosion by high-velocity

fluids entering the shell. The exchanger

manufacturer should be consulted to determine

whether or not such protection is required and

what form it should have.

• Protection is afforded by an impingement baffle

beyond the fluid inlet nozzle, a dome at the inlet

nozzle, or an oversized nozzle.

Vents and drains • Vents and drains, when removed, are usually

placed in the inlet or discharge piping to an

exchanger; however, they can be made to

connections on the inlet and outlet discharge

nozzles or to a boring through the tubesheet.

Tracing and

insulation

• Tracing and insulation are required to prevent

one or both sides from freezing unless a small

continuous flow is sufficient to prevent freezing

or it is permissible to drain the affected side of

the exchanger when it is not in use.

• Insulation may be required for heat or energy

conservation, personnel protection, or antisweat

reasons.

• Requirements for insulation and tracing are

indicated on a flow diagram as part of the

equipment description.

• It is not feasible to trace or insulate some

exchangers such as plate-type units. Shrouds

must be indicated for them for personnel

protection, and they must be drained when not

in use to prevent freezing.

Fans, blowers

and

compressors

Gas movers • An appropriate symbol should be entered on the

flow diagram.

Types of drivers • Gas movers are usually driven by electric motors.

• Compressors sometimes are operated by steam

turbines if excess steam is available or for

emergency use during an electrical interruption.

• Gasoline or diesel engines are employed for

emergency conditions or at remote locations.

Coolers • The heat developed in a compressor must be

removed if the discharge temperature exceeds the

unit’s maximum allowable value, usually about

177�C.

(Continued)



Item Notes

• Some compressors are designed to circulate

cooling water in jackets or internal passages.

Filtration is often required to prevent clogging if

municipal water is not used as the cooling

medium.

• Intercoolers are frequently used between

compressor stages with an aftercooler following

the unit; provision is made to remove

condensate.

• Large compressors usually have separate

lubrication systems including external

lubricating-oil coolers which require municipal

and filtered cooling water.

Knockout pots • These are required in the suction line to a

blower or compressor if there is liquid carryover

from the preceding step of the process or if

condensate can form in the suction piping.

Valved drains • Despite the use of knockout pots, moisture may

enter a blower or compressor and coalesce.

• Drains also are needed to remove wash fluid that

may be injected intermittently to clean the

blower or compressor blades.

Accumulators • Accumulators are needed to smooth suction gas

flow if the upstream volume is relatively small or

if there are upstream pressure variations.

• They are needed to smooth pulsations from a

reciprocating compressor and reduce downstream

pressure variation.

Screens, filters

and silencers

• A screen may be needed in the inlet line of an

air blower or compressor to prevent foreign

objects from entering the unit.

• A filter may be required if suction gas contains

small solid particles which would cause excessive

wear or a buildup of deposits. Non-lubricating

reciprocating compressors are particularly subject

to the abrasion of piston rings.

• Filters are used after oil knockout pots on

lubricated compressors to remove fine oil mists.

• Silencers are sometimes needed on the suction or

discharge sides of blowers or compressors to

reduce noise to an acceptable level. Some high-

speed centrifugals must be provided with a noise-

abatement envelope.

(Continued)



Item Notes

Controls and

interlocks

• Primary sensors and control valves are shown on

engineering flow diagrams to ensure economical

and satisfactory operation of blowers and

compressors and proper functioning of auxiliary

cooling and lubrication systems.

Vacuum

Equipment

Types of vacuum

equipment

• The particular type of vacuum equipment should

be represented by an appropriate symbol.

Drives and

motive forces

• Mechanical types of vacuum equipment are

usually operated by electric motors.

• Vacuum equipment with non-moving parts is

powered by a high-pressure fluid, usually steam,

air, or water. Piping for the motivating fluid is

shown on the flow diagram.

Cooling vacuum

equipment

• The heat of compression is removed from non-

wetted mechanical equipment by circulating

water through jackets or internal passages.

Interstage cooling is required if there is more

than one pumping stage.

• Liquid-ring vacuum pumps are cooled by a

modest flow of fresh water or a cooled recycle

and compatible sealant fluid. There is usually a

sufficient pressure differential between the

discharge and suction sides to accomplish the

recycle through an exchanger.

• Steam-jet ejector systems are supplied with

interstage condensers once the interstage dew-

point temperature is above that of the cooling

medium. Condensation may take place either by

direct contact with the cooling medium or

indirectly in a heat exchanger.

• Condensate from an interstage condenser is

subatmospheric and is collected through a closed

dip leg in a hot well or is pumped to disposal.

The flow diagram should include a note

regarding the minimum height of the interstage

condenser above the hot well, taking into

account the specific gravity of the continuous

phase of the condensate.

Knockout pots

and traps

• Inlet knockout pots are provided to prevent

entrained liquids from entering rotary, screw or

reciprocating-type vacuum pumps.

(Continued)



Item Notes

• Oil-lubricated units are followed by a knockout

pot to recover and recycle oil. Knockout pots

follow liquid-ring vacuum pumps to separate the

sealant from the discharge gas.

• Motive-steam lines to jet ejectors should contain

a condensate separator and be well trapped to

prevent intermittent condensate particles from

damaging the orifices in the jets.

Filters and

centrifuges

Types of filters

and centrifuges

• Representations of filters and centrifuges in an

engineering flow diagram should be typical of

their configuration.

Types of drivers • Centrifuges and rotating filters usually are

powered by electric motors; some centrifuges use

hydraulic motors to drive solids-removing plows.

• Compressed air or nitrogen is frequently used to

dislodge solids from filter socks in a baghouse or

to inflate flexible members in some plate-and-

frame filters for the compression of the cake or

to assist in its discharge.

Additives and

precoating

• Some liquid-solids separations require the

addition of surfactants or coagulants before

filtration.

• It is often necessary to precoat a filter or to mix

filter aid with the filter feed.

• All mix tanks, agitators, pumps, and controls are

to be shown on the flow diagrams as auxiliaries

to the filtration.

Recycle and

sampling

• A recycle line is included in batch operations to

ensure sufficient buildup of cake or filter

medium to give the desired clarity.

• A sampling point should be provided in a filtrate

discharge line if sampling ports are not provided

on the filter.

Auxiliary lines

and equipment

• A wash system is frequently required for filters

and centrifuges.

• A means to contain and transfer segregated solids

is needed.

• Provision must be made to collect and transfer

mother liquor from a centrifuge.

• Drum filters require vacuum equipment as an

auxiliary.

• Piping and equipment for all auxiliaries are

shown on the flow diagrams.

(Continued)



Item Notes

Piping A considerable amount of the work represented on engineering flow

diagrams is concerned with piping. The following are several important

aspects which should be kept in mind when preparing the diagrams.

Valves • Valves are used to isolate piping or equipment

from other portions of the process and to throttle

or divert flows; thus, they are an essential part of

any piping system. Valves should be designated

on engineering flow diagrams by a symbol which

not only shows where a valve is required but also

indicates the type of valve.

Bypasses for

control valves

• On-off valves, i.e. blocking valves, are usually

placed immediately upstream and downstream of

an automated control valve in continuous or

critical service along with a bypass line

containing a manual throttling valve about the

group. Such configuration permits continued

operation or orderly and safe shutdown of a

process in the event of a controller or control-

valve malfunction.

• The block valves and bypass line and valve

are usually of line size for main lines up to

75mm NB in diameter. For 75mm NB main

lines and larger, it is good practice to have the

bypass line and valve one line size smaller.

• Some self-regulating valves in clean service, such

as air-pressure regulators, are seldom provided

with a bypass line.

Vent and drain

sizes

• Vent lines on a vessel should be of at least the

size of the largest incoming recycled line.

Condensate traps • Vent lines or relief-valve discharge lines which

carry steam or condensable water vapor and have

a horizontal section that can form a seal leg

should be provided with a drain hole, if

permissible, or with a suitable condensate trap at

low points to prevent excessive pressure drops or

the development of excessive and dangerous

velocities of trapped liquids.

• A manual drain is usually suitable for intermittent

operations.

Protective screens • Atmospheric lines to the suction of a blower or

compressor as well as pump intake lines from natural

bodies of water or from sumps should be protected

by screens from the entrance of foreign bodies.

(Continued)



Item Notes

Jacketing and

bundling

• An outer pipe, known as a jacket, is sometimes

placed around a process line. An appropriate

heating or cooling utility stream flows in the

annulus to maintain the process fluid at a desired

temperature.

• Jacketing is shown on the body of the flow

diagram, and the word ‘jacketed’ is entered in

the tracing column on the line tabulation.

• A line is ‘bundled’ when it is placed next to a

hot or refrigerated line, and both lines are then

insulated together. The word ‘bundled’ is placed

in the tracing column of the line tabulation.

Boundary

definitions

• While building outlines per se are seldom

defined on engineering flow diagrams, it may be

necessary to locate a change point through which

a pipeline passes for tracing, insulation, piping

classification, or safety reasons.

• The transition between classifications is marked

by a short indicator perpendicular to the piping

line. The boundary definitions are delineated.

Instrumentation

and Safety

The process engineer enters all sensing instrumentation, control valves,

and safety devices on the P&ID or engineering flow diagram and

designates whether instrument measurements are to local, or remotely

indicated, or as recorded points. The instrument engineer completes

the control loops in the P&ID or on a separate instrumentation flow

diagram. The following subsections discuss some of the more common

instruments and safety devices to be considered for application on a

flow diagram. Other specialized items should be provided as required.

Relief valves • A relief valve, also known as a pressure safety

valve, protects equipment or piping from

pressures beyond its design maximum allowable

working pressure (MAWP).

• Typical situations for relief-valve application are:

x Any vessel, exchanger, column or other

equipment that can be completely isolated by

valving must be protected from an external

fire or runaway exothermic heats of reaction.

x Vessels with an open vent or overflow that is

of such size or length that excessive pressure

could be generated in the event of fire or

reaction excursion must be protected.

(Continued)



Item Notes

x Protection is also needed when the maximum

discharge pressure of a pump or compressor

feeding a piece of equipment or piping is

greater than its MWAP and the pump or

compressor can be deadheaded through the

equipment or piping.

x Exchangers need protection when a liquid

‘cold’ side is isolated and expanded owing to

the continued flow of the ‘hot’ side.

x A relief valve is usually placed after a pressure-

reduction valve to ensure that subsequent

equipment is protected in the event of a

malfunction of the reduction valve.

x If a relief valve may not reseat completely

owing to fouling by solids or gummy

materials, a pair of relief valves is used

in parallel and two ganged three-way

valves are incorporated in the piping so that

there is always one relief valve functioning

with the equipment while the other is being

cleaned.

• The sizes of relief-valve inlet and discharge lines

are shown on the flow diagrams. The discharge

side of most vapor safety valves is usually larger

than that of the inlet, while those in liquid

service usually have equal inlet and discharge

connections.

Rupture disks • A rupture disk is an alternative to a relief valve

when it is acceptable to allow pressure in

equipment or piping to fall and to lose material

until atmospheric pressure is reached.

• It consists of a frangible wafer of composite

materials or a thin metallic element which is

shattered or ripped apart at a predetermined

pressure.

• The material of the wafer and the metallic

element must be compatible with the fluid in the

equipment or piping. A thin membrane of

Teflon or other polymeric material is used to

prevent chemical attack of the disk and results in

an economical construction.

(Continued)



Item Notes

• A rupture disk can be placed ahead of a relief

valve to protect the relief valve from plugging or

to permit the valve to be constructed of more

economical materials. A pressure gauge is usually

placed between the relief valve and the rupture

disk to indicate the integrity of the disk.

Vacuum breakers • A vacuum breaker permits atmospheric air or a

compatible gas or vapor to enter a vessel at a

determined vacuum level to prevent collapse of

the unit at its maximum design pressure.

• Typical situations that require a vacuum breaker

are the withdrawal of liquid with insufficient

replacement of gas or vapor, the condensation of

vapors in an isolated piece of equipment

whereby the pressure in the vessel is reduced,

and the connection of vessels to a powered vent

system.

Conservation

vents

• A conservation vent is a combination of a relief

valve and a vacuum breaker.

• It normally allows storage vessels containing

volatile fluids to float within a limited pressure

range so that the loss of vapors is reduced.

Flame arresters • The vent on any unit containing an inflammable

fluid should be provided with a flame arrester to

prevent backflushing if the discharge vapors

become ignited.

• Conservation vents may be purchased with

integral flame arresters.

Pressure sensors • Pressure connections to equipment should be

made, where possible, in the gas or vapor space

to eliminate corrections for hydraulic head.

• Units such as filters, baghouses, heat exchangers,

or distillation columns which induce considerable

pressure drops often have a differential-pressure

measurement across the unit in addition to an

absolute- or gauge-pressure sensor at the inlet or

discharge.

• A pressure gauge on the discharge of a pump

provides information regarding the operation of

the pump; a permanent gauge (PI) may be

installed, or a valved provisional tap (PP) may be

used instead.

(Continued)



Item Notes

• Pressure gauges should be isolated from

equipment or piping with a suitable valve

or chemical seal. The latter is used with corrosive

fluids, toxic fluids, slurries, or fluids that would

solidify in the gauge. All isolating valves and

chemical seals are shown on the flow diagrams.

Temperature

sensors

• Whenever applicable, thermowells are located in

the liquid portion of a two-phase system to give

quicker responses.

• Thermowells, with or without indicators, are

frequently provided upstream and downstream of

a heat exchanger.

• It is good practice to provide a redundant

temperature measurement to activate an alarm

function in addition to the normal temperature

measurement used to control an exothermic

reaction.

Sight glasses and

lights

• A viewing port is often included in agitated

vessels or on distillation columns especially if

there may be foaming problems.

• If caustic or hydrogen fluoride is present, glass

ports must be protected by a suitable transparent

insert between them and the fluid.

• Sight glasses are available in multiple sizes and

configurations for use in piping. They are

employed to indicate the presence of liquid or an

interface, whether the liquid is stationary or

flowing (the turning of a wheel), and the

approximate flow rate (the lifting of a flapper).

• A sight glass is often accompanied by a sight

light; all sight glasses and lights are shown on the

flow diagrams.

Level indicators • All process vessels containing liquids require level

measurements.

• The simplest indicators are a shadowed liquid

level through the translucent wall of a plastic

tank with a calibration on the outside of the tank

or a calibrated dipstick for use as an intermittent

indicator in non-critical, non-hazardous service.

• A simple continuous-indicating device is a

window or a series of windows in the straight

side or a vertical cylindrical glass tube along the

side of a vessel. A series of tubes is required to

determine an interfacial level.

(Continued)



Item Notes

• A series of hollow external metal columns

provided with translucent reflux faces is used

when the application of glass windows or

cylinders is limited by the design pressure of the

vessel. This system is not suitable for showing

interfaces.

• Means must be provided to drain, flush and clean

external gauges.

• A pressure-sensing device may be used at or near

the bottom of the vessel as an indication of level.

A differential-pressure unit with one leg

connected to the vapor space is required if the

tank is not open or if its pressure in the vapor

space is greater than a few inches of water

column.

• A chemical seal is used to isolate the pressure

sensor from liquids which are corrosive or toxic,

could freeze at ambient temperatures, or are

slurries.

• A common level indicator uses a top-entering

dip tube through which a small metered flow of

compressed air, nitrogen or other compatible gas

is metered and bubbles trough the liquid. The

pressure of the gas, corrected for liquid density,

indicates the height of the liquid above the end

of the dip tube. A differential-type pressure

indicator is required for pressurized vessels.

• Bubble-type indicators should not be used with

saturated or nearly saturated solutions since

evaporation may leave deposits at the end of the

tube and lead to obstructions, causing erroneous

level readings.

• A guided float sensor gives a continuous level

reading of the surface of a liquid or a pile of

solids. It is also used to indicate a narrow-gauge

‘either-or’ local condition.

• Paddles, flappers, capacitance and sonic probes,

and radiation units are some of the devices

employed for specialized level indication. Strain

gauges are used to determine weight. Notes on

the flow diagrams are used to denote these

special level indicators.

(Continued)



Item Notes

Flexible

connections

• Equipment or piping made of ceramics, glass and

certain resinous materials is isolated by flexible

connections or hoses from damaging vibrations

caused by mechanical equipment, especially

reciprocating or high-speed types of gas movers.

• Flexible connectors are frequently used to ensure

that liquid lines of 200mm NB diameter or larger

do not overstress inlet or discharge nozzles on

pumps, filters, or similar equipment.

• Flexible connections serve to isolate a weigh tank

from its fixed entrance, discharge, or vent lines to

permit unimpeded movement of the vessel.

Expansion joints • An expansion joint is used in piping in lieu of

bends and loops to account for linear growth due

to changes in temperature.

• Differential linear growth between the two sides

of a heat exchanger is usually relieved by placing

an expansion joint in the shell unless the tubes

are coils or U tubes or the unit has a floating,

pull-through, or packed-head configuration.

• Consideration should be given to the extremes of

start-up or emergency conditions as well as

normal operating temperature in determining

whether or not an expansion joint is required.

Fire valves • These are ball or plug valves which maintain

their closure by a built-in spring mechanism but

are held open in normal operation by a link

mechanism containing a fusible section.

• They are recommended for installation at draw-

off nozzles on vessels with flammable contents.

Spring-closure

valves

• These valves are used when it is important that a

manually operated valve returns to a closed,

open, or partially open position after being used.

• They are often used instead of a restriction

orifice in services which tend to clog openings.

If the valve begins to clog, it can be opened fully

and then returned to a stop for its partially open

position.

Locked valves • Certain valves should remain in an open or

locked position until a change is authorized.

• The abbreviation LO or LC next to a valve on a

flow diagram notes that a valve is to be locked

open or locked closed.

(Continued)



Item Notes

Limit switches • Limit switches are required when the position of

a process or instrument valve must be known

before a particular operational step can be taken

or as part of an interlocked permissive sequence.

• Information is provided by small limit switches

on the valve housing. This is signified by using a

Y as the final identification letter in an

instrument bubble.

• Only one switch is required to know whether a

valve is fully open or fully closed. Two switches

are needed to know if the valve is either fully

open or fully closed.

Automatic

switch-over of

pumps

• Automatic switch-over is required in critical

services when a pumping operation must be

maintained despite mechanical or electrical

problems with the pump currently running.

• Isolating valves about the spare pump are kept

open and check valves provided in the discharge

line from each pump.

• A pressure-sensing device or flow switch in a

common discharge line detects a pump failure,

sounds an alarm, and automatically starts the

reserve pump.

• Pumps with severe but intermittent duties are

frequently placed on alternate start-up whereby

the pump started automatically is the one that

had not previously been operating.

Failure alarms • Current to motors of critical pumps,

compressors, agitators, or similar equipment is

sometimes monitored to indicate the status of the

process or of the equipment itself.

• Engineering flow diagrams should reflect the

presence of current indicators and accompanying

low or high alarms.

Interlocks • Often the operation of a piece of equipment

depends on another function in the process before

it can begin, continue or terminate its operation.

• The permissive conditions are covered by the

appropriate instrumentation or notes on the

engineering flow diagrams. Complex interlocks

are indicated by an appropriate symbol to show

that a separate logic diagram prepared by the

instrument engineer covers the instrumentation

and sequence of operations.

(Continued)



Item Notes

Miscellaneous Flow diagrams show many miscellaneous and specialty items to complete

a project. Some of the more frequently encountered features are

reviewed here:

Expansion tanks • Expansion tanks are required to relieve

thermal expansion when liquid-filled sections of

piping runs can be isolated and subsequently

heated.

• They may not be needed with water, as

developed pressure is relieved by minor

distortions of piping plus a small leakage at

gaskets or by the use of a thermal relief valve.

• They can be used to reduce water hammer or

induced shock waves for any fluid.

• Units are shown on a flow diagram with their

dimensions or as a note giving a make and model

number if they are to be purchased units.

Backflow

preventers &

air gaps

• Backflow preventers are required by authorities

supplying municipal water to prevent

contamination of the water supply.

• Backflow preventers or air gaps are used within a

plant to ensure that contaminants do not enter

the potable, locker-room or safety shower water

systems.

• A backflow preventer is a commercial item

consisting of a series of special check valves and

pressure-differential chambers to ensure the

absence of backflow. An air gap is an

arrangement whereby the line bringing

incipient process or plant water from the

municipal supply is terminated several inches

above the tank that is to supply the process of

plant water..

Safety showers,

eyewashes

• These are required whenever hazardous liquids

or solids are being unloaded or handled.

• They are usually purchased as a single

combination unit.

• Units are sized to give about 8m3/h to the

shower portion and 1.5m3/h to the eyewash for

a water pressure of 2 Bar. If the pressure is less, it

must be boosted, and it if is too great, units must

be supplied with the requisite restriction orifices.

(Continued)



Item Notes

• The units should be connected directly to the

potable water supply. If they are taken from a

pumped process- or plant-water supply, their

feed lines should be taken immediately after the

discharge of the process-water pump. A backflow

preventer isolates the shower-eyewash-water

takeoff point from any process or plant users.

• Units located outdoors must be protected from

freezing. This is accomplished by having an

underground water-supply assembly designed so

that water remains below the freezing level until

the unit is activated and residual water at the end

of use passes into a prepared drain at the base.

Aboveground supply piping is usually protected

by electric self-limiting tracing or electric

induction heating. The shower or eyewash unit

itself is then supplied with tracing or induction

heating.

• Safety-shower and eyewash units are shown on

engineering flow diagrams near the equipment

with which they are needed with a reference to

the utility flow diagram containing the

distribution header.

• Symbolic containers showing dilute acetic acid or

carbonate solutions are often shown next to an

eyewash to denote their presence for swabbing

purposes.

Utility service

stations

• It is good engineering practice to place utility

hose stations at convenient locations throughout

a plant. There should be air for such tasks as

blowing dirt or water from equipment,

unplugging or cleaning out lines, and operating

pneumatically driven mechanical equipment or

tools; water for housekeeping and emergency

cooling; and steam to thaw or clean lines and

equipment. Steam and water can be brought

together to form a separate hot-water station.

• Stations are sited on equipment-arrangement

drawings and given identifying numbers. Each

station is shown on the respective utility flow

diagram and identified by the assigned number.

(Continued)



Item Notes

• Air, steam and water pressure for normal usage is

limited to 2 Bar by pressure reducers to prevent a

pressure hazard caused by high-velocity fluid

impinging on the skin or by the whipping action

of hoses. High-pressure air required to drive

pneumatic motors or tools should have special

connectors.

• Hoses are typically 10 m long. If a series of hose

stations is required, they can be located

approximately 20 m apart.

Fire protection • Fire protection is required in operating and

storage areas if flammable ingredients are used in

the process or if gaseous or liquid fuels are

present.

• It is provided for control rooms, laboratories,

maintenance shops, and office areas.

• Although chemical canister units are provided,

fire hydrants with fire-water pumps and

underground fire-water mains are frequently

included. The number and location of fire

hydrants are set in accordance with local

ordinances.

• A method to reduce danger from a fire within a

large oil storage tank is to sparge steam into the

vapor space to displace air and snuff out the fire

by smothering it.

Seal fluid systems • Reference is made at appropriate seal or packing

locations on engineering flow diagrams to the

seal or flushing fluid system required at the

respective application point.

• A notation lists the flow diagram which presents

the distribution header and the line-identification

codes of lines bringing and returning the fluid.

• A separate section on a utility flow diagram

should illustrate a schematic of the typical

instrumentation and piping required at each

application point for the various systems, list the

various instruments and line numbers, and show

the distribution systems with pertinent

information in the line table.


APPENDIX 5

Teaching Practical ProcessPlant Design

INTRODUCTION

I cannot claim to have perfected design teaching though, after a few years of develop-

ment, our students can now produce deliverables which look like their professional

counterparts, and they understand how to work in teams to tight deadlines to produce

them. They also have an idea of what it is like to be an engineer, as I make the con-

text and frame of reference of the exercises as realistic as possible.

We teach process plant design from week 1 of year 1 in both the Chemical and

Environmental Engineering courses at Nottingham, in line with the IChemE’s wishes.

The presently commonplace approach of waiting until year 3 to ask students to design

something, having only received two years of noncontextualized natural science con-

tent, does not in my view work at all well. Many hours of guided and realistic practice

are what are needed for good basic competence.

The idea that process plant design is founded in natural science is given the lie by

the poor quality output of students who are set the challenge of a year 3 process plant

design based only upon math, science, and a copy of Sinnot & Towler. This is not

requiring them to carry out process plant design, this is requiring them to invent the

discipline from scratch.

Since their lecturers are expert in research rather than design, the traditional

approach produces a largely written document, with no recognizable engineering

deliverables, a smattering of rote hand calculations straight from Sinnot & Towler, or

worse still, a screen dump of a simulation program. Great emphasis may be given to

proper referencing of the primary literature sources the design is based on, in a docu-

ment which bears a strong resemblance to a research paper.

The students may well be asked to do the exercise in a group, but students and

staff often collude to make it into a linked set of individual exercises. The smartest

students will have designed the reactor, and the least able a few storage tanks or a heat

exchanger. No one will have designed a complete plant. On questioning, it will

become clear that none of the students understand anything about the process at a

whole system level, which was supposed to have been the whole point of the

exercise.

347

I have even seen process plant design courses which were assessed by formal exam-

ination, especially in the Far East. Process plant design ability cannot really be mea-

sured by traditional formal individual examination. It is too time-consuming, too

collaborative, and too creative to be measured using a technique which is good for

measuring an individual’s ability to recall facts and carry out rote calculation under

great pressure.

I prefer to assess a student’s ability to undertake a design exercise, which will be

collaborative in professional practice, by getting them to undertake collaborative

design exercises based on real plants I have designed.

There is, however, a potential difficulty with this approach which needs to be

managed—freeriders. Luckily for us, the UK Higher Education Academy’s

Engineering Centre financed the production of a dedicated software tool to help

with this.

PEDAGOGY

I am if anything less keen on pedagogic philosophizing than I am on the philosophy

of engineering, but there are a few useful ideas knocking about. These usually, how-

ever, need separating from the politicized and doctrinaire “theory” which they come

freighted with (even some philosophers agree that philosophy is a waste of time—see

Brennan in “Further Reading”).

I offer a series of papers in “Further Reading” for those who want to see

“research” backing of my teaching approach in peer-reviewed literature.

In summary, after investigation, I have concluded that pedagogic research is all too

frequently worthless, the introduction of ideas from pedagogic theory is usually coun-

terproductive, and there is not a single agreed fact in the entire field.

Educationalists may still be stuck at the stage of defining their terms and method-

ologies, but even a blind sow finds the odd acorn. Here are a few ideas which I have

found worthwhile if used with caution. Note, however, that to the extent that they

are useful they are also bleedin’ obvious.

X-based learning/XBL (where X represents a word like “problem,” “experience,”

or whatever) has come in and out of fashion since the nineteenth century, mainly

because it sounds like a great idea, accords with some people’s political leanings, and

students like it, but it basically doesn’t work.

It takes a lot of lecturer’s time to do well, maybe five times as much as a traditional

approach. It is therefore incredibly ineffective and inefficient as a teaching method,

with one possible exception, its use for teaching professional skills. I use it only for

this purpose.

“Constructive Alignment” suggests making as direct a link as possible between

teaching and assessment techniques and the thing being taught. If I want to know

348 Appendix 5: Teaching Practical Process Plant Design

how good a student is at designing plants, why not ask him to design a plant and assess

the output?

The CDIO (Conceive, Design, Implement, Operate) movement recognizes many

of the problems with engineering education which I do but, being pedagogues rather

than engineers, they mix the principles of liberal arts education and doctrinaire phi-

losophizing into their suggested solution to the problem. They have, however, rightly

identified many of the problems, and anyone wanting a research-based backing of my

suggestions could look to the website at http://www.cdio.org/.

“Good assessment practice”—I give an explicit explanation of learning objectives,

both at the start of the course and along with each coursework making it clear what I

am trying to have the students do. Each coursework also comes with a list of the rele-

vant “assessment criteria” (which have to be met to obtain a passing grade) and “grad-

ing criteria”, which describe how I will differentiate quality of work at grades above

the bare pass.

My grading criteria are broadly aligned with scales like the Bloom taxonomy

with one major difference. I do not reward the “extended abstract,” which I con-

sider an artifact of the discipline of those who devise such taxonomies. Piling sup-

position on top of philosophy on top of speculation is considered very clever

indeed in the liberal arts and humanities. Such a fragile approach is not, however,

well thought of in engineering. Rewarding it in my marks scheme would be

counterproductive.

“Good feedback” is very important in teaching a skill like design. Things men-

tioned in the pedagogic literature like avoiding empty praise and empty criticism, and

instead giving specific advice on how to do better next time, as well as explicit

guidance on how I am going to determine grade boundaries, improves student work

incrementally with each coursework. I do not allow my clear guidance on grade

boundaries to be an opening for mark negotiation. I am teaching design, not

negotiation.

“Formative assessment” sounds like a great idea to teach things like design which

require a lot of practice. The only problem I have found with it is its complete

impracticality. The vast majority of today’s students seem to be what are called “strate-

gic learners.” They basically won’t do a thing unless there are degree credits associated

with it, and they want the amount of marks available to correlate with the hours of

work pretty closely. I need to set marked assessments to get students to put the

required hours of practice in.

One last thing—as the paper by Prince in “Further Reading” shows, the lecturer’s

own research needs to be kept out of design teaching. Contrary to popular belief,

there is at best no correlation between research and teaching excellence. This is if

anything more true if the subject of your research is something you call “process

design.”

349Appendix 5: Teaching Practical Process Plant Design

http://www.cdio.org/

METHODOLOGY

My teaching essentially supports a series of group courseworks of increasing diffi-

culty. Working on these courseworks will take up the great majority of students’

time. I don’t do much traditional lecturing (and I’ll do less still when this book

comes out).

I have a teaching fellow with a lot of professional drafting experience who teaches

the students how to use AutoCAD and to some extent Excel in computer lab sessions.

Between the two of us there is a lot of contact time, around six hours per week, and I

might also see groups who are having difficulty in my office for another half an hour

a week.

I have classes of around 150, and I set three courseworks per module with maybe

six deliverables each. Group work allows me to reduce the amount of marking I have

to do to practical levels. Splitting the class into 20 groups reduces my marking load by

a factor of seven.

I set the coursework with tight deadlines, and I do not negotiate on workload,

group composition, deadlines, or marks. This allows me to get marks and feedback

back to students in a day or two, so that they will not repeat their mistakes on the

new coursework they will have received.

The UK’s Higher Education Academy produced a very useful and robust tool for

peer assessment called WebPA which I use to prevent free-riding in the group work

exercises.

I have a number of collaborating process plant designers who help to deliver lec-

tures, set assessments, and conduct design reviews with our students in years 1�3.

Our year 3 design project incorporates two group design reviews supervised by highly

experienced chartered engineers with decades of professional experience. The output

from all of our design courses is professional front end engineering design (FEED)

study documentation; and design reviews aim to produce professional quality docu-

ments (albeit early-career version).

There are a number of modules which have to be integrated with the process

design module for best results, most notably process control, materials, fluid mechan-

ics, and separation processes modules.

We have also stopped teaching students how to use modeling and simulation pro-

grams until year 3, and do not allow their use in year 3 projects. Early access to simu-

lation programs has the same effect on process design ability as access to calculators

has on mental arithmetic.

As I said in the last section, this is not a time-efficient approach. It takes up a

lot of my time and a lot of the students’ time. But, as I am making them into engi-

neers, I think it is worthwhile for all of us.


EXERCISES

I have included in this section the exercises I use in my design teaching. All the cour-

seworks are real plants I (or in one case a visiting professional) have actually designed.

I have not included model solutions, as I recommend this text to my students, and

in any case there are no right answers. Even my own design solutions to these pro-

blems are not “right answers,” as they were based on the state of the art at the time. I

would do them differently (and better) today.

I have, however, included along with the courseworks my marking criteria (which

I give to students along with the question when setting coursework) which explain to

both students and lecturers what I am after from the exercises.

1. Class exercise—The marshmallow challenge

2. Class exercise—A 5 m3 tank

3. Coursework 1—Ortoire water treatment plant

4. Coursework 2—Alnwick Castle water feature

5. Coursework 3—Upgrading Jellyholm water treatment plant

6. Class Exercise—Fun size creativity

7. Coursework 4—Groundwater pilot plant

8. Coursework 5—Supercritical water oxidation plant

9. Class exercise—Pharmaceutical Intermediate Bulk Container (IBC)

10. Coursework 6—Pharmaceutical aerosol manufacture

Class exercise: The marshmallow challenge“The task is simple: in eighteen minutes, teams must build the tallest free-standing

structure out of 20 sticks of spaghetti, one yard of tape, one yard of string, and one

marshmallow. The marshmallow needs to be on top.”

See http://marshmallowchallenge.com for details and supporting video.

A 40” tower is thought to be the world record, 20” is average.

Class exercise: A 5 m3 tankVery early in the course, students are asked to give the optimal height to diameter

ratio for a tank for liquid with a 5 m3 capacity made of 3 mm thick stainless steel.

Though they know all the necessary mathematics to solve the problem, they will

struggle to make the necessary assumptions, and to frame the problem in a way which

allows it to be solved using mathematics.

The instructor should not give them clues, or even tell them whether it has an

open top or bottom unless asked.


http://marshmallowchallenge.com

There are very different solutions based on which assumptions are made, and some

sets of assumptions, while themselves reasonable, yield no useful mathematical solutions.

Students who complete the exercise should be asked to comment on how many

significant figures are meaningful in the answer if 3 mm stainless steel costs d2/m2.

The problem is very useful for demonstrating the difference between engineering

and math, and for introducing sensitivity analysis to an audience of mathematically

competent engineering incompetents like freshers (or university lecturers).

Coursework 1: Ortoire water treatment plantTask1. Produce, as individuals, a single tab Excel spreadsheet which shows a rough initial

design for a water treatment plant taking 15MLD of water from a lake at

750 mAOD, passing it through settlement tanks on to pressure sand filters, and

from there into a chlorine contact tank to TR60 of 30 min HRT and 1 m water

depth at 575 mAOD. Distance from lake to CCT is 1,500 m. Fall on ground is

constant along this 1,500 m.

2. Numbers and diameters for circular settlement tanks and sand filters, external and

internal dimensions of CCT, and theoretical and actual pipe diameters sizes will be

needed to be calculated as a minimum.

3. Produce a single Excel file for your group which combines all of your individual

submissions, with the submission you have as a group agreed is the best at the front.

4. Use your calculated sizes of units to produce a group scale drawing showing as

much detail as you can in both plan and elevation.

5. Peer assess each other using WebPA.

Rules of thumb for design• Surface loading settlement tanks 1 m/h

• HRT settlement tanks 2 h

• Max diameter settlement tanks 30 m

• Surface loading sand filters 15 m/h

• Sand filter depth 2 m

• Max diameter sand filters 3 m

• Static head required for sand filter operation 10 m

• Superficial velocity pipework 1.5 m/s

TLAs• AOD—above ordnance datum

• HRT—hydraulic residence time

• CCT—chlorine contact tank

• MLD—megaliters per day


Learning outcomes

D1: Define a problem and identify constraints.

D2: Design solutions according to customer and user needs.

S1: Knowledge and understanding of commercial and economic context of chemical/

environmental engineering processes.

P1: Understanding of and ability to use relevant materials, equipment, tools, processes, or

products.

P5: Ability to use appropriate codes of practice and industry standards.

P8: Ability to work with technical uncertainty.

Assessment criteria

1. Apply British Standards

2. Size unit operations via Rules of Thumb

3. Undertake hydraulic calculations

4. Produce appropriate plant layout drawing (plan and elevation)

Grading criteria

ALL Assessment Criteria passed to a satisfactory level 3RD

Produce Excel spreadsheet which sizes unit operations and pipework with evidence of

some analysis. Integrate into system design and produce a drawing to BS that clearly

shows design intent.

2:2


detailed analysis. Integrate into system design and produce a drawing to BS that

clearly shows design intent. Cost, safety, and robustness are considered.

2:1


critical analysis. Integrate into system design and produce a detailed drawing to BS

that clearly shows design intent. Cost, safety, and robustness are analyzed.

1ST

Teaching Notes: You can make the exercise harder by specifying the quantity of water produced rather than thequantity input, requiring an iterative solution to the mass balance.

Coursework 2: Alnwick Castle water featureYou have been engaged to provide hydraulic and process design of a new water fea-

ture at Alnwick Castle by Company X, on behalf of the Duchess of Northumbria.

Your responsibility is to make sure that the water in the feature is suitably treated,

and that the required effects are achieved.

The drawing provided shows the Grande Cascade, which is the centerpiece of the gar-

dens (YouTube has plenty of videos of it in action). Hidden underneath the cascade are the

plant rooms containing the pumps and treatment plant which make the feature work.

Your job is to design the required plant and pumps, and fit them in into the plant rooms.

You need to submit your calculations as an Excel spreadsheet, a P&ID, and a mod-

ified version of the GA provided (note that all working water levels are to be marked

on the GA), along with manufacturers datasheets for any equipment used.


The Company X Water Features Design Manual provided should facilitate your

design, but additional marks are valuable for a more sophisticated approach. (I wrote

the design guide to allow plumbers to design water features—I expect more from you.)

Learning outcomes


D2: Design solutions according to customer and user needs.S1: Knowledge and understanding of commercial and economic context of chemical/

environmental engineering processes.P1: Understanding of and ability to use relevant materials, equipment, tools, processes, or

products.P5: Ability to use appropriate codes of practice and industry standards.


Assessment criteria


2. Size unit operations via Rules of Thumb3. Undertake hydraulic calculations

4. Produce appropriate plant layout drawing (plan and elevation)5. Produce P&ID

Grading criteria



some analysis. Integrate into system design and produce drawings to BS that clearlyshow design intent.

2:2

Produce Excel spreadsheet which sizes unit operations and pipework with evidence ofdetailed analysis. Integrate into system design and produce a drawings BS that clearly

show design intent. Cost, safety, and robustness are considered.

2:1


critical analysis. Integrate into system design and produce detailed drawings to BSthat clearly show design intent. Cost, safety, and robustness are analyzed.

1ST

Teaching notes: Students will be able to find out quite a lot of data on this real

project on the internet. A good solution will involve taking publicly available data on

flows and appearance and flowrates calculated in a number of ways to size the pumps

and treatment plant.

Coursework 3: Upgrading Jellyholm water treatment plantScenario: Jellyholm water treatment worksThe Jellyholm water treatment plant in Sauchie (FK10 3AZ; Figure A5.1) is fed with

water from the nearby Gartmorn reservoir (Grid Ref NS 92000 94200). The reservoir


is subject to periodic algal blooms, and you have been commissioned to upgrade the

plant to handle these.

The plant feeds housing which has lead piping, and orthophosphate dosing is

to be provided to prevent lead dissolving into the drinking water on its way to supply.

The assignment is to carry out as detailed a design as you can of an upgrade to the

treatment works. You can if you wish follow what was done as shown on the case

study document, or you can offer an alternative approach.

Deliverables1. P&ID

2. GA

3. Hydraulic Calculations, from dam to treated water tank

4. Process Design Calculations including Mass Balance

5. Control Philosophy

6. Approximately 2,000-word proposal for your plant upgrade including capital and

running cost estimate, and justification of design choices

7. WebPA assessment of yourself and fellow group members

For avoidance of doubt:

All drawings are to be in AutoCAD format. All spreadsheets are to be in MS

Excel. Report in MSWord. Your drawings should be to British Standards. They

should be neat, clear, and accurate. Your report should also be neat, clear, concise,

and accurate.

If you don’t understand any part of this assignment, ask me to explain. If you can-

not see how to do some or all of the task set, ideally you would ask among your

group first, then see if you can find the answer by research, then ask me.

Figure A5.1 Jellyholm water treatment works. Copyright Image reproduced courtesy of DoosanEnpure Ltd.


Learning outcomes





P1: Understanding of and ability to use relevant materials, equipment, tools, processes, or products.



Assessment Criteria



3. Undertake mass balance and hydraulic calculations


5. Produce P&ID

6. Produce Proposal

7. Produce Control Philosophy

Grading criteria



some analysis. Integrate into system design and produce drawings to BS that clearly

show design intent. Produce Control Philosophy which explains control system design

intent. Produce proposal document which explains all proposed refurbishments.

2:2


detailed analysis. Integrate into system design and produce a drawings BS that clearly

show design intent. Produce Control Philosophy which clearly explains control

system design intent, and adequately considers most common system disturbances.

Produce proposal document which clearly explains all proposed refurbishments.

Cost, safety, and robustness are considered.

2:1


critical analysis. Integrate into system design and produce detailed drawings to BS

that clearly show design intent. Produce Control Philosophy which clearly explains

control system design intent and adequately considers many system disturbances.

Produce proposal document which clearly explains all proposed refurbishments.

Cost, safety, and robustness are analyzed.

1ST

Coursework 4: Groundwater pilot plantA temporary groundwater treatment plant has been running for some time on a site,

and it is time to propose a permanent replacement. A pilot trial has been undertaken

to see if alternative technologies to that used for the temporary plant are viable.


Using the temporary and pilot plant data provided, choose a suitable mixture of

technologies, and produce a conceptual design with GA and P&ID appropriate to the

site shown on the drawing provided.

It should be noted that the approach used on the real plant may not be the optimal

one based on the data provided to you.

DeliverablesExcel Spreadsheet with as a minimum mass balance, unit operation sizing, and


AutoCAD General Arrangement Drawing.

AutoCAD P&ID.

Learning outcomes





P1: Understanding of and ability to use relevant materials, equipment, tools, processes, or products.



Assessment criteria


2. Analyze data provided to choose technologies


4. Undertake hydraulic calculations


6. Produce P&ID

Grading criteria


Produce Excel spreadsheet which analyses data, sizes unit operations and pipework with

evidence of some analysis. Integrate into system design and produce drawings to BS

that clearly show design intent.

2:2


evidence of detailed analysis. Integrate into system design and produce drawings to

BS that clearly show design intent. Show how cost, safety, and robustness have been

considered.

2:1


evidence of critical analysis. Integrate into system design and produce detailed

drawings to BS that clearly show design intent. Show how cost, safety, and robustness

have been analyzed.

1ST


Class exercise: Fun-size creativityAfter a short lecture on creative systems including the McMaster 5-point strategy, stu-

dents are invited to solve a problem which proved very difficult in practice—produc-

ing “fun size” Mars bars. The problem is that the Mars bar filling sticks to the blades

used to cut it up into small pieces.

Coursework 5: Supercritical water oxidation plantTaskProduce, as individuals, multitab Excel spreadsheets which show a rough initial design

for a supercritical water oxidation (SCWO) nanoparticle production plant comprising

as a minimum the following elements, as shown on the PFD supplied:

1 No. DI water stock tank, dosing pump, and valves delivering up to 3 m3/h

1 No. hydrogen peroxide stock tank, dosing pump(s), and valves delivering up to

1 m3/h

1 No. metal salt 1 stock tank, dosing pump(s), and valves delivering up to

1.5 m3/h

1 No. metal salt 2 stock tank, dosing pump(s), and valves delivering up to 1 m3/h

1 No. capping agent stock tank, dosing pump(s), and valves delivering up to 1 m3/h

1 No. electrical process heater

1 No. preheat heat exchanger

1 No. reactor

1 No. energy recovery heat exchanger

1 No. postfilter

2 No. cooling water pumps each delivering 6.5 m3/h

Cooling tower

Cooling water filter

1. Sizes of these elements and actual pipe diameters will be needed to be calculated

as a minimum.

2. Produce a single Excel file for your group which combines all of your individual

submissions, with the submission you have as a group agreed is the best at the front.

3. Use your calculated sizes of units to produce individual scale drawings (GA) show-

ing as much detail as you can in both plan and elevation, choose one to represent

the group, and submit all as a single file with the chosen submission clearly marked.

4. Produce an individual P&ID showing as much detail as you can, choose one to

represent the group, and submit all as a single file with the chosen submission

clearly marked.

5. Peer assess each other using WebPA.


Learning outcomes





P1: Understanding of and ability to use relevant materials, equipment, tools, processes, or

products.



Assessment criteria



3. Undertake mass and energy balance and hydraulic calculations


5. Produce P&ID

Grading criteria



some analysis. Integrate into system design and produce drawings to BS that clearly

show design intent.

2:2


detailed analysis. Integrate into system design and produce a drawings BS that clearly

show design intent. Cost, safety, and robustness are considered.

2:1

Produce Excel spreadsheet which sizes unit operations and pipework with

evidence of critical analysis. Integrate into system design and produce detailed

drawings to BS that clearly show design intent. Cost, safety, and robustness are

analyzed.

1ST

Class exercise: Pharmaceutical intermediate bulk container (IBC)You are considering the design of a new dispensary for a solid dose facility. The

following formulation is to be dispensed into IBCs within the dispensary.

Material Quantity to beadded (kg)

Bulk density(kg/l)

Occupationexposure limit

Lactose mono SDS 45.0 0.625 10 mg/m3

Lactose 220.0 0.625 10 mg/m3

Hydroxy cellulose 15.0 0.500 10 mg/m3

Carboxy methyl

cellulose

23.0 0.500 10 mg/m3

Micronized active

ingredient

30.0 0.150 5 μg/m3


Figure A5.2 gives the typical dimension of the IBC that the client wishes to use.

The dimensions shown are fixed and cannot be changed. This configuration is based

on filling the IBC so that the top of the repose-cone of powder does not go any

higher than the top of the straight section of the IBC. Calculate the length of straight

side shown and hence the overall height of the IBC. Show your working and

assumptions.

Teaching Notes: This is like a more sophisticated version of the 5 m3 tank exercise,

which I give to second year and MSc students as a preparation for coursework 6.

Even if you give them the formulae and simplifying assumptions below, they tend to

make quite heavy weather of it.

1,100 square

125

100

450

850

250

Cone of powder at theangle of repose.

Volume given by Eq. 1

To becalculated

Calculate volumeof base-cone from

Eq. 2

Figure A5.2 IBC.


It can be interesting to use figures rounded to 2�3 significant figures at the start of

the exercise, to illustrate the problems of premature rounding.

The volume of repose-cone can be calculated from:

Volume5 1=3ðBase Area3HeightÞ ðA5:1Þ

The volume of base-cone of the IBC can be calculated from:

Volume5 1=3ðA1 1A2 1OðA13A2Þ3HeightÞ ðA5:2Þwhere A15 the cross-sectional area of the top of the base-cone and A25 the

cross-sectional area of the bottom of the base-cone. Assume that the bottom of the

base-cone has a square cross section and ignore the transition to the circular outlet.

Also assume that this square has the same length and breadth as the outlet valve

diameter.

Coursework 6: Pharmaceutical aerosol manufactureGeneral requirements

No. Description Acceptance criteria

1. Product contact surfaces—metallic

materials of construction.

Stainless steel type EN 1.4435

2. Product contact surfaces—

polymeric materials

PTFE or PVDF compliant with ASME

BPE 2012 Part PM


elastomeric materials

Perfluorelastomer, silicone, or PTFE

encapsulated EPDM compliant with

ASME BPE 2012 Part PM


materials surface condition.

Compliant with ASME BPE 2012

Table SF-2.2-1


materials surface finish.

Electro-polished to 0.38 μm. Compliant

with ASME BPE Table SF-2.4-1


nonmetallic materials surface

condition.

Compliant with ASME BPE 2012

Table SF-3.3-1

7. Material certificate of compliance. EN 10204 Type 3.1

8. Product contact piping. OD hygienic tubing—True imperial

dimensions

9. Product piping couplings Hygienic union couplings compliant with

EN 11864

10. Product contact liquids and clean

steam—valves

Weir-type diaphragm compliant with

ASME BPE SG-2.3.1.2

Note: Product contact surfaces include all clean utilities downstream of the final sterilizing grade filter, propellantdownstream of the final sterilizing grade filter, and clean in place systems.


Dispensary operations


1. Active ingredient maximum water

content

2% w/w

2. Active ingredient maximum

ethanol content

1% w/w

3. Active ingredient drying time. 15 h maximum

4. Active ingredient dryer end point 80�C at 20 mbar abs

5. Active ingredient dryer yield .98% mass basis

6. Active ingredient median particle

size

2 μm mass basis

7. Active ingredient milling yield .98% mass basis

8. Active ingredient CIP fluid Purified water BP

9. Active ingredient CIP fluid flow 35 l/min/m of vessel circumference with a

static spray device

10. Active ingredient CIP final rinse

liquid.

Purified water BP

11. Active ingredient dispensing

accuracy

6 0.1% mass basis

12. Excipient dispensing accuracy 6 0.5% mass basis

13. Lubricant maximum water

content

1% w/w

14. Sweetener maximum water

content

0.5% w/w

Flavor maximum water content 0.5% w/w

Propellant storage and distribution


1. Design pressure—maximum Vapor pressure at maximum design

temperature plus 1 bar g

2. Design pressure—minimum Full vacuum

3. Design temperature—maximum 50�C4. Design temperature—minimum 220�C5. Tank capacity Tanker load plus 5,000 l

6. Temperature of propellant transferred to

manufacturing

20 6 1�C

7. Transfer rate to manufacturing To take no more than 20 min

8. Level indication accuracy 6 1%

9. Pressure relief requirements Duplex pressure relief valve

10. Pump head Static head 10 m

Piping equivalent length 300 m

Heat exchanger pressure drop max 0.5 bar


11. Pump NPSH To avoid cavitation at all times

12. Piping upstream of sterilizing filter—

metallic material

Stainless steel type EN 1.4404

13. Pump mechanical seal Sealess magnetic seal


nonmetallic material

PTFE or EPDM


piping standard

Flanged to BS 4504


valves

Flanged Ball Valves

17. Propellant filtration Prefilter 5 μmFinal filter 0.2 μm sterilizing

grade filter

Formulation


1. Design pressure maximum Vapor pressure at maximum design temperature

plus 1 bar g

2. Design pressure minimum Full vacuum

3. Design temperature maximum 100�C4. Design temperature minimum 220�C5. Maximum batch size 1,000 l

6. Minimum batch size 500 l

7. Vessel nominal volume To the top of the heat transfer surface

8. Vessel minimum stirred volume 50 l maximum. Batch shall remain homogeneous

at minimum stirred volume

9. Formulation temperature 20 6 1�C10. Dispersion time 10 min at 400 rpm

11. Mixing requirement To achieve full suspension

To achieve efficient heat transfer

12. Heating requirement Hold at 20�C during formulation and can filling

Hold at 80�C during CIP and drying

13. Cooling requirement Cool to 20�C after drying in 30 minutes

14. In-place cleaning fluid 2% w/w NaOH in purified water BP

15. In-place cleaning final rinse Purified water BP

16. In-place cleaning temperature 80 6 1�C17. In-place cleaning fluid flow 35 l/min/m of vessel circumference with a static

spray device

18. Heat transfer method External dimpled jacket covering shell and

bottom head

19. Maximum pressure drop through

heat transfer jacket

0.5 bar g

20. Maximum variation of propellant

concentration during can

filling

6 1% of propellant concentration


21. Maximum water concentration in

final formulation

0.5% w/w

22. Maximum oxygen concentration

in final formulation

0.1% w/w

23. Maximum nitrogen concentration

in final formulation

0.1% w/w

24. Vessel drainability Fully drainable with no pools greater than 5 mm

diameter after draining

25. Mixing dead spots No mixing dead spots, vessel surface shall be

flush below the liquid level including the

bottom outlet

Can filling


1. Can purge 2 can volumes minimum

2. Filling temperature 20 6 1�C3. Filling rate 30 cans per minute

4. Rejects 2% maximum

5. Samples 1 per 1,000 cans

6. Overspray after filling head removed 1 off can valve capacity

7. Function testing 3 valve actuations

8. Maximum air concentration in suspension in the can 1% w/w

19. Maximum water content in the suspension in the can 1% w/w

10. Operating shift pattern 3 shifts per day

Start Monday 6 pm

Finish Friday 10 pm

11. Formulation maximum hold time 40 h

12. Air extraction rate each can filling booth 1,000 Nm3/h

13. Air extraction rate each function tester 1,000 Nm3/h

Deliverables• Full mass balance and energy balance

• Preparation of detailed PFD covering the whole process

• Detailed P&ID to industry standards covering equipment in group in No. 3 above

• Equipment sizing for all components

• GA

Equipment sizing should include• Operational size, for example, liters, liters/min, kW, etc.

• Design capacity as above but with design margin added

• Approximate physical dimensions

• Materials of construction


• Design pressure

• Motor power

• NPSH for pumps, etc.

Complete industry standard data sheet for the formulation vessel, homogenizer,

and agitator.

Blank data sheet to be provided.

Learning outcomes

D1: Define a problem and identify constraints.D2: Design solutions according to customer and user needs.

S1: Knowledge and understanding of commercial and economic context of chemical/environmental engineering processes.

P1: Understanding of and ability to use relevant materials, equipment, tools, processes, orproducts.

P5: Ability to use appropriate codes of practice and industry standards.P8: Ability to work with technical uncertainty.

Assessment criteria

1. Apply British Standards2. Size unit operations via Rules of Thumb

3. Undertake mass and energy balance and hydraulic calculations4. Produce appropriate plant layout drawing (plan and elevation)

5. Produce P&ID

Grading criteria


Produce Excel spreadsheet which sizes unit operations and pipework with evidence ofsome analysis. Integrate into system design and produce drawings to BS that clearly

show design intent.

2:2


detailed analysis. Integrate into system design and produce a drawings BS that clearlyshow design intent. Cost, safety, and robustness are considered.

2:1

Produce Excel spreadsheet which sizes unit operations and pipework with evidence ofcritical analysis. Integrate into system design and produce detailed drawings to BS

that clearly show design intent. Cost, safety, and robustness are analyzed.

1ST

FURTHER READINGBiggs, J.B., 2003. Teaching for Quality Learning at University, second ed. Open University Press/Society

for Research into Higher Education, Buckingham.Brennan, J., 2010. Scepticism about philosophy. Ratio 23 (1), 1�16.Hattie, J., 2009. Visible Learning; A Synthesis of over 800 Meta-analyses Relating to Achievement.

Routledge, London.Loddington, S., Pond, K., Wilkinson, N., Wilmot, P., 2009. A case study of the development of WebPA:

an online peer-moderated marking tool. Br. J. Educ. Technol. 40 (2), 329�341.Marzano, R.J., 1998. A Theory-Based Meta-analysis of Research on Instruction. Mid-continent

Research for Education and Learning, Aurora, CO.













Nicol, D.J., Macfarlane-Dick, D., 2006. Formative assessment and self-regulated learning: a model andseven principles of good feedback practice. Stud. High. Educ. 31 (2), 199�218.

Orsmond P. (2004) Self- and peer-assessment: guidance on practice in the biosciences. Higher EducationAcademy Centre for Bioscience: Teaching Bioscience: enhancing learning series.

Prince, M.J., Felder, R.M., Brent, R., 2007. Does faculty research improve undergraduate teaching? Ananalysis of existing and potential synergies. J. Eng. Educ. 96 (4), 283�294.

Strivens, J., 2007. What theory should we use—if any? Interpreting “scholarship” on programs for newuniversity teachers. PRIME 2 (2), 81.

The CDIO Initiative. Available from: ,http://www.cdio.org/..Victoria University of Wellington, 2004. Group Work & Group Assessment. University Teaching Development

Centre (Online). Available from: ,http://www.utdc.vuw.ac.nz/resources/guidelines/GroupWork.pdf..










http://www.cdio.org/

http://www.utdc.vuw.ac.nz/resources/guidelines/GroupWork.pdf

GLOSSARY

There are a number of words and phrases which are not in common use and, more problematically, there are

a number which are in common use, but mean different things to different people. In this glossary, I define

the sense in which I am using them in this book. This is intended to be the most commonly held meaning.

Basic Engineering Design Data (BEDD) Information compiled to allow conceptual design in petro-

chemical sector

Best Practice The consensus heuristics of practitioners

Block Flow Diagram (BFD) Academic approximation of a PFD

BS British Standard

CAD Computer-aided design/drawing

Capex Capital expenditure

Conceptual Design The initial stage of design; content varies between sectors

Consultancy A company which offers advice, and rarely progresses design beyond conceptual stage

Contracting Company A company which contracts to build plants, and usually does its own detailed design

Control and Instrumentation Engineer A hybrid chemical/software engineer or sometimes instru-

ment technician found mainly in petrochemical industry operating companies

CoV Co-efficient of variance

DCS Distributed Control System

DEFRA UK Department for Environment, Food & Rural Affairs

Deliverables Things delivered under a contract; in a plant design context, mainly drawings

Design Basis Information compiled to allow design at any stage, in general terms. See BEDD for a

sector-specific exception

Design Philosophy Accounts of decisions on how a number of common design problems and issues

will be handled during a design; best generated early in the design process

Designer Someone who designs a plant and, in the context of this book, is willing to be legally respon-

sible for it

Dimensioned Drawing Drawing marked with dimensions of real-world counterparts of items illus-

trated. Not guaranteed to be a scale drawing

Draffie Draughtsman/woman

Due Diligence Generating sufficient certainty in your opinions, considering the potential downside if

you are wrong

dxf Drawing Exchange Format, a file format developed by Autodesk (authors of AutoCAD) which

allows usually less than perfect file sharing with other CAD programs

EA UK Environment Agency

EPC Engineering, Procurement and Construction company, aka “Contracting Company”

Engineering The profession of imagining and bringing into being a completely new artifact which

safely, cost�effectively, and robustly achieves a specified aim

Engineering Science The application of scientific principles to the study of engineering artifacts

FEANI “Federation Internationale d’Associations Nationales d’Ingenieurs”

FEED Front End Engineering Design—an initial design exercise

Functional Design Specification (FDS) A description in carefully chosen words of what a plant

designer wants the software to do

General Arrangement (GA) Drawing A scale drawing which shows the layout in space of a plant;

aka a “plot plan” among other things

HAZASS Hazard Assessment: Mecklenburgh’s hazard evaluation/identification technique

367

HAZOP Hazard and Operability Study

HMI Human�machine interface

HSE Health Safety Environment or UK Health and Safety Executive

IPPC Integrated Pollution Prevention and Control

ISO International Standards Organization

Mogden Formula A formula used in the United Kingdom to calculate trade effluent charges

Natural Science The activity of trying to understand natural phenomena (cf. Engineering Science)

Olfactorithmetic The ability to detect an implausible numerical answer by “smell”

Operating Company A company whose main activity is managing and operating process plants

Opex Operating expenditure

Optimization Improving a process by balancing a number of variables against cost, safety, and robustness

Partial Design An academic approximation of parts of the design process which falls far short of total design

(qv)

PERT Program/project evaluation and review technique. A program evaluation tool, allied with critical

path analysis

Pinch Analysis A largely academic exercise to minimize resource usage

Piping and Instrumentation Diagram (P&ID) The process engineer’s signature drawing, showing

physical and logical interrelationships between process plant components

Piping Engineer Specialist in piping and sometimes plant layout used in some industries and countries

to produce plant layouts

PLC Programmable Logic Controller—Industrial Computer

Plot Plan See GA

Precision To do with whether the instrument will give me the same reading against the same true value

the next time I test it (though I do use it in other senses in the book)

Problem A design problem will require the use of engineering judgment and imagination to solve, as

data and/or design methodologies are lacking

Process Design An abstract conceptual “design” of a chemical process with no real consideration of

cost, safety, or robustness

Process Flow Diagram (PFD) A drawing which represents the mass balance, resembling in many

ways a simplified P1 ID

Process Intensification A largely academic conception of combining unit operations, or Trevor Kletz’s

term for what is now usually called minimization in inherent safety techniques

QA Quality Assurance

Repeatability Precision under tightly controlled conditions over a short time period

Reproducibility Variability over time

RTFM (In polite terms) Kindly Read the Manual

SCADA System control and data acquisition

Scale Drawing Drawing whose dimensions are consistently some ratio of the size of their correspond-

ing real-world counterparts

State Of The Art The set of heuristics of a designer or designers

Task A design task involves using a well-established methodology and robust data to grind out an obvi-

ous answer

Tiffie Instrument technician

T’Internet The World Wide Web in the vernacular of the North of England

TLA Three-letter acronym—engineering joke

Total Design “Total Design is the systematic activity necessary, from the identification of the market/

user need, to the selling of the successful product to satisfy that need—an activity that encompasses

product, process, people and organization.”—Stuart Pugh

Validation Ensuring software matches reality

Verification Ensuring software is free of coding errors

WOD Write-only documentation

368 Glossary

INDEX

Note: Page numbers followed by “f” and “t” refer to figures and tables, respectively.

AAcademic costing practice, 165�167

capital cost estimation by MPI/factorial method, 165

economic potential, 166

net present value (NPV), 166

operating cost estimation, 166

payback period, 166

sensitivity analysis, 166�167

Academic HAZOP, 258

Academic myopia, 255

Academic “process integration,” downside of, 274�275

capital cost of “integrated” plants, 274

commissioning “integrated” plants, 274�275

maintaining “integrated” plants, 275

Academic versus professional practice, 13�18

Accuracy, 179

Accurate capital cost estimation, 167�171

bought-in electrical items, 168

bought-in mechanical items, 168

civil and building works, 169

competitive design and pricing, 171

design consultants, 169

electrical installation, 168�169

man-hours estimation, 170

margins, 170

mechanical installation, 168

pricing risk, 170

project programming, 170

software and instrumentation, 169

Accurate operating cost estimation, 171

Aesthetics and process plant layout, 210�211

Aftermarket systems, 151

Afterthought safety, 82

Agitators, checklist for, 324t

Alarm overload, 263

American Petroleum Institute (API) standard, 85

American Society of Mechanical Engineers (ASME) standard, 85

AMS Realtime, 65

Antoine equation, 118

Applied mathematics, 5, 10

process plant design as form of, 72

Applied science, 6

Approximations, of process plant, 17�18

Architects, 210�211

Art of engineering, 277�278

“As low as reasonably practicable” (ALARP), 217

Aspentech Hysys, 63

Autodesk AutoCAD, 53, 65�66

Autodesk Simulation CFD, 67

Automatic control, 177�180

control systems, specification of, 180

instrumentation, specification of, 178�179

accuracy, 179

cost and robustness, 179

precision, 178�179

safety, 179

BBackwash control, 186�187

CAD representation of, 186f

Basic Engineering Design Data (BEDD), 21�22

Batch distillation, 97

Batch heat transfer, 96�97

Batch processing, 94�96

apparent simplicity, 94�95

batch integrity, 95�96

flexibility, 95

nonsteady state, 96

solids handling, 95

Batch reaction, 97

Batch sequencing, 97�98

Bentley Systems Microstation, 66

Bernoulli’s equation, 115

Best efficiency point (BEP), 125

Best engineering practice, 18�19

Biology, 281

Block Flow Diagram (BFD), 37, 201, 255�256

Blowers, 138�139, 184, 324t

Blowout panels, 238�239, 238f

Boiler Explosions Act (1882), 36

Boilerplate, 36

British jet engine, 93

Bursting discs, 238, 238f

CCabling, 147�148

Calculations, design, 48�50

Capital cost estimation

accurate, 167�171

by MPI/factorial method, 165

Casio FXCG20, 58

Categories, of design, 56�57

energy balance, 57

hydraulic design, 56�57

mass balance, 57

unit operation sizing and selection, 56

CDIO (conceive, design, implement, operate) movement, 8, 349

Central services, 214

Centrifugal compressors, 184, 184f

Centrifugal pump, 120, 122, 192, 192f, 314t

CHAZOP. See Hazard and Operability (HAZOP) studies

369

Checklists for engineering flow

diagrams, 323, 324t

Chemical cleaning control, 187�188, 187f

The Chemical Engineer, 44

“Chemical process design,” 32�33, 72�73

Chemically enhanced backwash (CEB), 187�188

Chemstations CHEMCAD, 63

Civil and building works, 44, 169

Civils/buildings partners, consultation with, 250

Client documentation, 84

Colebrook�White approximation, 113, 120

Commercially minded engineers, 84

Commissioning, 212�213

Communication, 31, 249

Competitive design and pricing, 171

Complicated/fragile versus simple/robust design, 98�100

estimation/feel, 100

lessons from the slide rule, 99�100

Components and materials of process plants, designing and

selecting, 129

electrical and control equipment, 145�151

cabling, 147�148

control system, 149�151

instrumentation, 148

Motor Control Center (MCC), 145�147

matching design rigor with stage of design, 130�131

materials of construction, 131�138, 132t

scaling and corrosion, 133�138

mechanical equipment, 138�144

heat exchangers, 143�144

pumps/blowers/compressors/fans, 138�139

valves, 140�142

process plant design engineer, 129�130

Compressibility, 120

Compressors, 138�139, 184, 184f, 324t

Computer Aided Process Engineering (CAPE) of Institution of

Chemical Engineers (IChemE), 19, 53, 62

Computer Fluid Dynamics (CFD)

study, 67, 121

Computer-aided design (CAD) representation, 51, 55, 226

of actuated valve control, 196f

of backwash control, 186f

of break tank filling and emptying, 195f

of centrifugal compressor, 184f

of centrifugal pump, 192f

of chemical cleaning control, 187f

of distillation, 185f

of dosing pump, 194f

of dry running protection, 190f

of fired heater/boiler, 188f

of heat exchanger, 189f

of no flow protection, 191f

of positive displacement pump, 193f, 194f

Computer-aided drawing/drafting, 65�66

Autodesk AutoCAD/Inventor, 65�66

Bentley Systems Microstation, 66

PTC Creo, 66

Computer-aided hydraulic design, 67

Autodesk Simulation CFD, 67

COMSOL Multiphysics, 67

Matlab/Simulink, 67

Microsoft Access, 68

Microsoft Visio, 67�68

Computer-aided process design, 66

Computers

computer-aided design (CAD), 65�66

drawing/drafting, 65�66

hydraulic design, 67

process design, 66

modern design tools, implications of, 54�56

use, by chemical engineers, 54

use and abuse of, 19�20

COMSOL Multiphysics, 64, 67

Conceive, design, implement, and operate (CDIO)

movement, 8, 349

Conceptual design, 21�23, 105�106, 163, 212

of chemical processes, 23�25

emulation, 44

issues, 284�285

modeling as, 24�25

Conceptual design stage, 17, 24, 32, 35, 80, 105, 130, 176,

219�221

formal safety studies, 219

human factors, 220

inherent safety, 219�220

user-friendly design, 220�221

Conceptual layout, of process plant, 212�214

central services, 214

for construction, 216

construction, commissioning, and maintenance, 212�213

detailed layout methodology, 215�216

earthworks, 214

emergency provision, 213

indoor/outdoor, 212

materials storage and transport, 213

methodology, 214�215

security, 213�214

wind positions, 212

Conceptual/FEED fast-tracking, 30�31

Construction, 212�213

design for, 107

lack of knowledge of many materials of, 262

materials of, 131�138, 132t

scaling and corrosion, 133�138

for process plant layout, 216

Construction Design and Management (CDM)

regulations, 9, 203

“Constructive Alignment,” 348�349

Continuous stirred tank reactor (CSTR), 97

Continuous versus batch design, 94�98

batch processing, using, 94�96



flexibility, 95

solids handling, 95

main batch design requirements, 96�98

batch distillation, 97

batch heat transfer, 96�97

370 Index

batch reaction, 97

batch sequencing, 97�98

energy balance and utility requirements, 98

nonsteady state, 96

Control of Major Accident Hazards (COMAH) legislation, 208,

212, 218�219

Control of Substances Hazardous to Health (COSHH)

legislation, 218

Control system, 149�151

aftermarket systems, 151

DCS system, 150�151

local controllers, 149�150

programmable logic controllers (PLCs), 150

specification of, 180

supervisory control and data acquisition (SCADA), 150

Coolidge, Calvin, 280

Corrosion, scaling and, 133�138

Corrosion table, 134t

Cost estimation, 42�44

academic approach, 42�43

classes of, 164t

professional budget pricing, 44

professional firm pricing, 44

Costing, 163, 204

academic costing practice, 165�167

economic potential, 166

MPI/factorial method, capital cost estimation by, 165

net present value (NPV), 166

operating cost estimation, 166

payback period, 166

sensitivity analysis, 166�167

basic, 164�165

implications for cost, 81

“integrated” plants, capital cost of, 274


plant layout, 209�210

professional costing practice, 167�171

accurate capital cost estimation, 167�171

accurate operating cost estimation, 171

DDangerous Substances and Explosive Atmospheres Regulations

(DSEAR), 232

Darcy�Wiesbach equation, 120�121

Datasheets, 46�47

DCS system, 150�151, 180

Dearden, Harvey, 249

Deliverables, 35

cost estimate, 42�44

academic approach, 42�43

professional budget pricing, 44

professional firm pricing, 44

datasheets, 46�47

design basis and philosophies, 35�36

design calculations, 48�50

equipment list/schedule, 45

functional design specification (FDS), 40

general arrangement, 40�41

isometric piping drawings, 51

layout drawing, 40�41

piping and instrumentation diagram, 38�40

plot plan, 40�41

process flow diagram (PFD), 37

project program, 42

safety documentation, 47�48

HAZOP study, 47

zoning study/hazardous area classification, 47�48

simulator output, 52

specifications, 36

Design, 6�7

Design basis and philosophies, of process plant, 35�36

Design calculations, 48�50

Design envelope

lack of consideration of, 256

setting, 100�102

summary statistics, 102

Design manuals, 85

of process plant, 17

Design methodologies, 90�91

Design/procurement fast-tracking, 31

Detailed design, 26�27, 105, 153

Detailed design fast-tracking, 31

Detailed design stage, 36, 130, 215, 221

Detailed layout methodology, 215�216

Direct on Line (DOL) starters, 145

Distillation, 185


Dosing pump, 182, 194


Drawing, 26, 35, 41, 54�55

engineering, 37

General Arrangement (GA), 40

isometric piping, 51, 51f

layout, 40�41

P&ID, 38

Drinking water plants, 111

Dry running protection, 190


Duct chart, 119f

EEarthworks, 214

Economic potential, 43, 166

Electrical and control equipment, 145�151

cabling, 147�148

control system, 149�151

aftermarket systems, 151

DCS system, 150�151

local controllers, 149�150

programmable logic controllers (PLCs), 150

supervisory control and data acquisition (SCADA), 150

instrumentation, 148

Motor Control Center (MCC), 145�147

Electrical area classification distances

for centrifugal pumps, 314t

for equipment other than pumps, 314t

Electrical/software partners, consultation with, 250

Emergency provision, 213

371Index

Emergency shutdown valves, 241�242, 241f

Energy balance, 57, 111�112

and utility requirements, 98

Engineer tensions in design, 103�105

iron triangle, 105

risk aversion, 104�105

Engineering, 5�6

literature of, 279

Engineering design, 7, 72

Engineering Equipment & Materials Users’ Association

(EEMUA), 181

Engineering science, 6, 16

Engineers, more experienced, 86

Envirowise, 267�268

Equipment datasheet, 46f

Equipment knowledge, lack of, 259�261

pump types and characteristics, 259

valve types and characteristics, 259�261

Equipment list/schedule, 45

Equipment suppliers, 56, 143

consultation with, 250

Errors of beginner to avoid, 255

academic “HAZOP,” 258

academic myopia, 255

excessive novelty, 255

lack of attention to detail, 255�256

lack of consideration

of construction, commissioning, and nonsteady state

operation, 256

of design envelope, 256

of natural stages of design, 255

of needs of other disciplines, 255

of price implications of choices, 258

of processes away from core process stream, 257�258

lack of knowledge, 259�261

of many materials of construction, 262

of many types of unit operations, 261�262

of pump types and characteristics, 259

of valve types and characteristics, 259�261

lack of redundancy for key plant items, 257

lack of utilities, 262

layout, 262�263

2D, 262

lack of control rooms and MCCs, 263

lack of room and equipment for commissioning and

maintenance, 262

online resources, uncritical use of, 258�259

parallel and series installation, 257

process control, 263�264

alarm overload, 263

lack of isolation for instruments, 263

lack of redundancy for key instruments and safety

switches, 263

measuring things, 263

P&ID notation, 264

Excessive novelty, 255

FFans, 138�139, 324t

Fast-tracking, 30�33

to bad design, 32�33

conceptual/FEED, 30�31

design/procurement, 31

FEED/detailed design, 31

Feedstock, 110�111, 267

and product specifications, 111

Filters, 186�188

backwash control, 186�187

chemical cleaning control, 187�188

Filters and centrifuges, checklist for, 324t

Financial risks, 164

Fired heaters/boilers, 188�189

First principles design, 17, 48, 71�72, 83�84

Flammability hazards, 232�233

Flare stacks, 242�243, 242f

Formal design reviews, 251�252

interdisciplinary design review, 251

safety engineering review, 252

value engineering review, 251�252

Formal methods, 273�274

pinch analysis, 273�274

safety, 222�229

functional safety standards, 227�228

HAZAN, 226

HAZASS, 222�223

HAZID, 222

HAZOP, 223�224

Professional HAZOP procedure, 224�226

safety integrity level/LOPA, 228�229

sustainability, 229�230

IChemE metrics, 229�230

Formal safety studies, 219

“Formative assessment,” 349

Front end engineering design (FEED), 25�26, 31, 350

Functional design specification (FDS), 40, 176

Functional safety standards, 227�228

Future of process plant design, 69

changes in, 74�75

“chemical process design,” 72�73

first principles design, 71�72

as form of applied mathematics, 72

heuristic design, 71�72

network analysis, 73

primary research as the basis of engineering design, 72

process porn, 69�71

process simulation replacing the design process, 73�74

GGantt chart, 42, 42f

Gas detectors, 240, 241f

Gases, 118�121, 119f

General Arrangement (GA), 40�41, 41f, 79�80, 105, 118

German Junkers engine, 92�93

Goal Seek, 113, 120

“Good feedback,” 349

Graphical calculators, 58

Gubbins, 29, 155

HHaaland’s equations, 120

Handheld calculators, 58

372 Index

Handling facilities for equipment, 306t

Hazard Analysis (HAZAN) studies, 219, 226

risk matrix, 226

Hazard and Operability (HAZOP) studies, 12, 26, 208

academic, 258

methodology, 223�226

Hazard Assessment (HAZASS), 213, 222�223

Hazard Identification (HAZID) studies, 219, 222

Health Safety Environment (HSE), 204�205, 208, 217

Health/safety/environment, 204�205

Heat exchangers, 133, 143�144, 144t, 189, 236


checklist for, 324t

Heuristic design, 48, 71�72, 80

Human�machine interface (HMI), 150, 180

Hydraulic calculations, 49, 80, 115

hydraulic networks, 121�122


Computer Fluid Dynamics (CFD), 121

Moody diagram, 119

nomograms, 116�118

spreadsheet method, 120�121

superficial velocity, 116

pump curves, 122�125, 122f

Hydraulic design, 56�57

computer-aided, 67

Hysys, 12, 53, 63

IIndoor/outdoor plant, 212

Informal data exchange, 253�254

Informal design reviews, 250�251

civils/buildings partners, consultation with, 250

electrical/software partners, consultation with, 250

equipment suppliers, consultation with, 250

peers/more senior engineers, consultation with, 251

Ingress protection (IP) ratings, 235

Inherent safety, 218�220

Institution of Chemical Engineers (IChemE), 3, 11, 31�32, 53,

62, 72�73, 217, 220, 229�230

Computer Aided Process Engineering (CAPE) of, 19, 53, 62

Instrumentation, 38, 148, 180�199

and safety, 324t

safety-critical, 179

selection, 148t

software and, 169

specification of, 178�179

Instrumented Protection Functions

(IPFs), 228

Integrated design example, 283

and process control example, 284�288

conceptual design issues, 284�285

dosing issues, 286

integrated solution, 287�288

layout/piping issues, 285

price issues, 286

robustness issues, 287

safety issues, 286

“Integrated” plants

capital cost of, 274

commissioning, 274�275

maintaining, 275

Integrating design, 268�274

formal methods, 273�274

intuitive method, 268�272

Intellectual knowledge, 279

Interdisciplinary design review, 251

Interesting versus boring design, 92�94

Intermediate design, 106�107

Internet, 87, 258

Intuitive method, 268�272

Invensys SimSci Pro/II, 63

Iron Triangle, 105

ISO 9000 series, 252�253

Isometric piping drawings, 51

Iterative calculations, using Excel for, 113

JJunkers Jumo 004 engine, 93f

LLack of attention to detail, 255�256

Langelier index (LSI), 138

Larson-Skold index, 138

Layout, of process plant, 201

and aesthetics, 210�211

affecting factors, 204�207

cost, 204

health/safety/environment, 204�205

man-made environment, 207

natural environment, 206

regulatory environment, 207

robustness, 205

site selection, 205�206

for construction, 216

and cost, 209�210

general principles, 202�204


conceptual layout, 212�214

detailed layout methodology, 215�216

and safety, 208�209

Layout drawing, 40�41, 41f

Least capital cost, 81

Libraries, 87

Linear actuators, 198

Liquefied, flammable gases

preliminary minimum distances for, 309t

Liquefied oxygen, preliminary minimum distances for, 308t

Liquids, 117�118, 120

stored at ambient temperature and pressure, 311t

Local controllers, 149�150

Long-term exposure limit (LTEL), 233

LOPA, 228

Lotus 123, 59

MMain Plant Items (MPIs), 105, 165

MPI costs, 165

Maintenance activities, 212

Manager/engineer tensions in design, 103�105

373Index

Manager/engineer tensions in design (Continued)


the iron triangle, 105

Man-hours estimation, 170

Man-made environment, 207

Manufacturer’s catalogues and representatives, 85�86

Manufacturers’ literature, design from, 155

Mass and energy balance, 109

Excel tab, 112�113

feedstock and product specifications, 111

iterative calculations, using Excel for, 113

recycles, handling, 111�112

stages of plant life, 111

unsteady state, 109�110

Mass balance, 57

Matching design rigor with stage of design, in process plant

layout, 212�216

Materials storage and transport, 213

Mathematics, 5

applied, 5, 72

precision in, 178

MathWorks, 62

Matlab, 53, 62

Matlab/Simulink, 67

Mechanical equipment, 138�144

heat exchangers, 143�144

pumps/blowers/compressors/fans, 138�139

valves, 140�142

Mecklenburgh methodology, 222�223

Microscopic resolution, 265

Microsoft

Access, 68

Excel, 53, 59�61, 60f, 65, 112�113, 119

for iterative calculations, 113

Project, 53

Visio, 67�68

Visual Basic, 61

Mixers, checklist for, 324t

Mobile devices, 57�58

Molecular biology, 281

Moody diagram, 56, 115, 119

Motor Control Center (MCC), 145�147, 146f, 168, 263

MPC (multivariable predictive control), 151

Multipurpose plants, 95

NNatural environment, 206

Natural stages of design, lack of consideration

of, 255

Net positive suction head (NPSH), 118

Net present value (NPV), 166

Network analysis

forming the core of design practice in future, 73

process simulation replacing the design process in future,

73�74

New design tools, implications of, 102�103

Nomograms, 116�118, 117f

Numerical analysis software, 62

OOddo-Tomson index, 138

Online resources, uncritical use of, 258�259

Operating cost estimation, 166

accuracy, 171

Operation and maintenance (O1M) manuals, 176�177

Optimization of plant design, 265

academic “process integration,” downside of, 274�275

capital cost of “integrated” plants, 274

commissioning “integrated” plants, 274�275

maintaining “integrated” plants, 275

high feedstock use/waste, 267

high utilities usage/waste, 267

integrating design, 268�274

formal methods, 273�274

intuitive method, 268�272


process integration, 266

Oracle Primavera, 65

Outdoor plant, 212

PParallel and series installation, 257

Partial design, 13

Payback period, 166

PC software, 59�68

MathWorks MATLAB, 62

Microsoft Visual Basic, 61

MS Excel, 59�61

numerical analysis software, 62

PTC Mathcad, 62

spreadsheets, 59�61

Peers/more senior engineers, consultation with, 251

Personal and process safety, 230�231

Personal sota, 280�282

Philosophy of engineering, 278�279

“Physics porn,” 69

PID (Proportional, Integral, Differential) controllers, 149, 149f

Pilot plant trials/operational data, 86

Pinch analysis, 273�274

downside of, 274

Pipe flow chart nomogram, 117f

Pipeline design calculations, 50f

Piping and Instrumentation Diagram (P&ID), 37�40, 79�80,

96, 112, 140, 201, 264

Plant life, stages of, 111

Plant separation tables, 301

electrical area classification distances for centrifugal pumps,

314t

electrical area classification distances for equipment other than

pumps, 314t

handling facilities for equipment, 306t

liquefied, flammable gases, preliminary minimum distances

for, 309t

liquefied oxygen, preliminary minimum distances for, 308t

liquids stored at ambient temperature and pressure, 311t

plots and sites, preliminary general spacings

for, 303t

374 Index

preliminary access requirements at

equipment, 304t

preliminary electrical area classification distances, 313�321

definitions, 313�321

preliminary extent

double-walled tank, 320f

for a fixed roof tank, 320f

opentopped oil/water separator, 321f

quench drain channel or effluent interceptor pit, 321f

single-walled tank, 320f

of zones 0, 1, and 2 in open-topped constructions, 321f

of zones 1 and 2 for a floating roof

tank, 321f

of zones 1 and 2 for road or rail, 319f

of zone 1 for drum filling in open, 321f

of zone 2 around a liquid sample point, 318f

of zone 2 around a relief valve, 318f

of zone 2 in compressor house, 316f

of zones to outside building, 317f

preliminary minimum clearances at

equipment, 305t

preliminary tank farm layout

elevation, 312f

plan view, 312f

site areas and sizes, 302t

size of storage piles, 322

tank farm layout, preliminary spacings for, 308�312

Plot plan, 40�41

Plots and sites, preliminary general spacings for, 303t

Political risks, 164

Positive displacement pumps, 193, 193f

Posthandover redesign, 28

Practice of engineering, 279�280

Precision, 178�179

Preliminary access requirements at equipment, 304t

Preliminary electrical area classification distances, 313�321

Preliminary extent of zone 1

for drum filling in open, 321f

for floating roof tank, 321f

in open-topped constructions, 321f

for road or rail, 319f

Preliminary extent of zone 2

around a liquid sample point, 318f

around a relief valve, 318f

in compressor house, 316f

double-walled tank, 320f

for a fixed roof tank, 320f

for a floating roof tank, 321f

opentopped oil/water separator, 321f

quench drain channel or effluent interceptor

pit, 321f

for road or rail, 319f

single-walled tank, 320f

of zones 0, 1, and 2 in open-topped constructions, 321f

of zones to outside building, 317f

Preliminary minimum clearances at equipment, 305t

Preliminary tank farm layout

elevation, 312f

plan view, 312f

Pressure relief valves (PRV), 237

Previous similar plants, 86�87

Process control, 263�264

alarm overload, 263

lack of isolation for instruments, 263

lack of redundancy for key instruments and safety switches,

263

measuring things, 263

P&ID notation, 264

Process control system, designing, 175

automatic control, 177�180

specification of control systems, 180

specification of instrumentation, 178�179

matching design rigor with stage of design, 176

operation and maintenance manuals, 176�177

specification of operators, 177

standard control and instrumentation strategies, 180�199

alarms, inhibits, stops, and emergency stops, 180�181

chemical dosing, 182�183

compressors/blowers/fans, 184

distillation, 185

filters, 186�188

fired heaters/boilers, 188�189

heat exchangers, 189

pumps, 190�194

tanks, 195

valves, 196�199

Process design

“is” and “ought” of, 91�92

versus process plant, 11�13

Process Flow Diagram (PFD), 25�26, 37, 74, 79�80, 118, 203,

255�256

Process Integration, 79

Process Intensification, 79, 266

Process optimization, 84

Process plant

academic versus professional practice, 13�18

approximations, 17�18

best engineering practice, 18�19

computers, use and abuse of, 19�20

design, 5, 9�10

design, meaning of, 6�7

design manuals, 17

engineering, meaning of, 5�6

engineering design, 7

professional judgment, 18

project life cycle, 8�9

standards and specifications, 16

state of the art, 18�19

thumb rules, 17

versus castles in the air, 15

versus process design, 11�13

Process plant design engineer, 129�130

Process porn, 69�71

Process risks, 164

Product engineering, 29

Professional costing practice, 167�171

accurate capital cost estimation, 167�171

accurate operating cost estimation, 171

375Index

Professional design methodology, 89

batch distillation, 97

batch heat transfer, 96�97

batch processing, 94�96



flexibility, 95

nonsteady state, 96

solids handling, 95

batch reaction, 97

batch sequencing, 97�98

conceptual design, 106

design for construction, 107

energy balance, 98

interesting versus boring design, 92�94

intermediate design, 106�107

manager/engineer tensions in design, 103�105

iron triangle, 105


new design tools, implications of, 102�103

process design, “is” and “ought” of, 91�92

right versus wrong design, 92

setting the design envelope, 100�102

simple/robust versus complicated/fragile design, 98�100

understanding a design, importance of, 103

utility requirements, 98

variations on a theme, 107

whole-system design methodology, 105�106

Professional judgment, 18

Professional practice, 249

formal design reviews, 251�252

interdisciplinary design review, 251

safety engineering review, 252

value engineering review, 251�252

general design methodology, 249

informal data exchange, 253�254

informal design reviews, 250�251

civils/buildings partners, consultation

with, 250

electrical/software partners, consultation with, 250

equipment suppliers, consultation with, 250

peers/more senior engineers, consultation with, 251

quality assurance and document control, 252�253

Programmable logic controller (PLC), 150, 180, 283

Project life cycle, 8�9

Project management/programming tools, 64�65

AMS Realtime, 65

Microsoft Excel, 65

Microsoft Project, 65


Project program, 42

Project schedule. See Project program

PTC Creo, 66

PTC Mathcad, 62

Pump curves, 122�125, 122f

complex, 124f

intermediate, 123f

Pumps, 138�139, 190�194

centrifugal, 192

checklist for, 324t

dosing, 194

dry running protection, 190, 190f

lack of equipment knowledge, 259

no flow protection, 191, 191f

over-temperature protection, 191

positive displacement, 193

selection, 138t, 139t

QQ/H curve, 123

Quality assurance and document control, 252�253

Quench tanks, 244

RReal rules of thumb, 83

Recycles, handling, 111�112

Regulatory environment, 207

Resource efficiency measures

for cleaning and washdown, 270t

for process plant, 269t

Revolutionary design, 92

Right versus wrong design, 92

Risk aversion, 104�105

Robustness, 205

implications for, 82

Rotary actuators, 197

RTO (real-time optimization), 151

Rule of thumb design, 83, 153�154

Ryback, Casey, 280

Ryznar/Carrier Stability Index (RSI), 138

SSafety, implications for, 82

Safety and process plant layout, 208�209

Safety devices

emergency shutdown valves, 241�242

flare stacks, 242�243

gas detectors, 240

overpressure protection, 235�239

blocked in (hydraulic expansion), 237

blowout panels, 238�239

bursting discs, 238

burst tube case, 236

closed outlets (on vessels), 236

cooling water/medium failure, 236

exterior fire case, 237

pressure relief valves, 237

quench tanks, 244

scrubbers, 243

specification of, 235

static protection, 240

under-pressure protection, 239

vacuum relief valve, 239

water sprays, 244

Safety documentation, 47�48

HAZOP study, 47

zoning study/hazardous area classification, 47�48

376 Index

Safety engineering review, 252

Safety integrity level/LOPA, 228�229

Scale-up and scale-out, 156�157

Scaling and corrosion, 133�138

Schoolboy errors, 259

Science, 6

Scotty Principle, 104�105

Scrubbers, 243, 243f

Security, 213�214

Sensitivity analysis, 164�167

Separation processes, 157�160, 158t

Sewage and industrial effluent treatment plants, 111

SFAIRP (so far as is reasonably practicable), 217

Short-term exposure limit (STEL), 233

Shutdown valves (SDVs), 241

Simple/robust versus complicated/fragile design, 98�100

estimation/feel, 100

lessons from the slide rule, 99�100

Simulation programs, 62�64

Aspentech Hysys, 63



design by, 84, 154�155


Simulator output, 52

Site areas and sizes, 302t

Site redesign, 27�28

Site selection, 205�206

Small-scale batch solids handling equipment, 95

Solver, 113, 122

Sources of design data, 84�87, 156

client documentation, 84

design manuals, 85

internet, 87

libraries, 87

manufacturer’s catalogues and representatives, 85�86

more experienced engineers, 86

pilot plant trials/operational data, 86

previous designs, 86�87

standards, 85

Specification of equipment with safety implications, 230�235

access, 231�232

flammable, toxic, and asphyxiant atmospheres, 232�235

confined space entry, 234�235

Dangerous Substances and Explosive Atmospheres

Regulations (DSEAR), 232

flammability hazards, 232�233

ingress protection (IP) ratings, 235

toxic hazards, 233

personal and process safety, 230�231

principles, 230�235

Specifications, 36

of operators, 177

Spreadsheets, 59�61, 120�121

Stages of plant life, 111

Stages of process plant design, 21

conceptual design, 21�23

chemical process, 23�25

modeling as, 24�25

detailed design, 26�27

fast-tracking process, 30�33

conceptual/FEED fast-tracking, 30�31

design/procurement fast-tracking, 31

fast-track to bad design, 32�33

FEED/detailed design fast-tracking, 31

front end engineering design (FEED)/basic design, 25�26

general, 21

posthandover redesign, 28

product engineering, 29

site redesign, 27�28

unstaged design, 29

Standard control and instrumentation strategies, 180�199

alarms, inhibits, stops, and emergency stops, 180�181

chemical dosing, 182�183

actuated valve control, 183

pump speed control, 182�183

pump stroke length control, 183

compressors/blowers/fans, 184

centrifugal, 184

positive displacement, 184

distillation, 185

filters, 186�188

fired heaters/boilers, 188�189

heat exchangers, 189

pumps, 190�194

tanks, 195

valves, 196�199

Standards and specifications, of process

plant, 16

Star Delta starters, 145

State of the art, 18�19

Static protection, 240

Storage piles, size of, 322

Stuff, 29

Stuxnet, 150

Superficial velocity, 116

Supervisory control and data acquisition systems, 110, 150

unsteady state SCADA screen, 109�110, 110f

Sustainability, 229�230

IChemE metrics, 229�230

System control and data acquisition, 283

System level design, 77

cost, implications for, 81

first principles design, 83�84

matching design rigor with stage of

design, 80�81

putting unit operations together, 79�80

robustness, implications for, 82

rule of thumb design, 83

safety, implications for, 82

simulation program, design by, 84

sources of design data, 84�87

client documentation, 84

design manuals, 85

internet, 87

libraries, 87

manufacturer’s catalogues and representatives, 85�86

more experienced engineers, 86

377Index

System level design (Continued)

pilot plant trials/operational data, 86

previous designs, 86�87

standards, 85

TTank farm layout, preliminary spacings for, 308�312

Tanks, 195

Teaching practical process plant design, 347

exercises, 351

methodology, 350

pedagogy, 348�349

Three-letter acronym (TLA), 36

Thumb rules, of process plant, 17

TI Nspire, 58

“T’Internet,” 87

Tools, of process plant design, 57�58

graphical calculators, 58

handheld calculators, 58

mobile devices, 57�58


MathWorks MATLAB, 62

Microsoft Visual Basic, 61

MS Excel, 59�61

numerical analysis software, 62

PTC Mathcad, 62

spreadsheets, 59�61

project management/programming tools, 64�65

AMS Realtime, 65

Microsoft Excel, 65

Microsoft Project, 65


simulation programs, 62�64

Aspentech Hysys, 63




Total Design, 13, 79

Toxic hazards, 233

Twenty-first century process plant design tools, 53

chemical engineers, use of computers by, 54

computer-aided design (CAD), 65�66

drawing/drafting, 65�66

hydraulic design, 67

process design, 66

design, categories of, 56�57

energy balance, 57

hydraulic design, 56�57

mass balance, 57

unit operation sizing and selection, 56

modern design tools, implications of, 54�56

tools, 57�58

graphical calculators, 58

handheld calculators, 58

mobile devices, 57�58


project management/programming tools, 64�65

simulation programs, 62�64

UUnder-pressure protection, 239

Understanding a design, importance of, 103

Unit operation sizing and selection, 56

Unit operations, designing, 153

first principles design, 154

manufacturers’ literature, design from, 155

matching design rigor with stage of

design, 153

rule of thumb design, 153�154

scale-up and scale-out, 156�157

separation processes, 157�160

simulation program, 154�155

sources of design data, 156

Unstaged design, 29

Upset conditions, specific, 289t

User-friendly design, 220�221

Utility requirements, 98

VVacuum equipment, checklist for, 324t

Vacuum relief valve, 239

Value engineering review, 251�252

Valves, 140�142, 142t, 196�199

lack of equipment knowledge, 259�261

linear actuators, 198

rotary actuators, 197

selection, 141t

valve positioner/limit switch, 198�199

Variable speed drive (VSD), 145

Variations on a theme, 107

Vessels, checklist for, 324t

WWater sprays, 244, 244f

Well-integrated design, 81

“What-if analysis” tools, 113

Whittle W2-700 engine, 93f

Whole-system design methodology,

105�106

Wind positions, 212

“Write only” documentation (WOD), 36

XX-based learning (XBL), 348

378 Index